feat: M1 模拟器内核 — SplitMix64 RNG + 确定性 step + 对拍测试 + headless 冒烟工具 + Desktop 自动演示

[变更性质]
本 commit 为功能开发，交付设计文档 §2 模拟器内核（里程碑 M1），非缺陷修复或结构重构。

[新增功能]
- Kernel：Rng.fs 提供 SplitMix64 纯函数 RNG（ofSeed / nextUInt64 / nextFloat32），World 携带种子状态。
- Kernel：Sim.fs 重写为 struct record（Vec2/Avatar/Input/TimeStep/World），step : World → TimeStep → World 纯函数，dt 固定 1/60 由内核常量给定，Time = Tick * dt 不做浮点累加；initialWorld 由无参改为带 seed。
- Tests：MSTest 确定性对拍——同 seed 轨迹逐 tick 相等、异 seed 发散、step 纯度、长跑有限/有界/时间-刻度关系、SplitMix64 已知向量、nextFloat32 值域。
- Headless：--days N --seed S 冒烟工具，每 1000 tick 输出摘要行，统计 NaN/越界计数并决定退出码。
- Desktop：LV_AUTOPLAY=1 环境变量启用正弦驱动自动演示，窗口标题与启动日志标注模式。

[实现方案]
- 全部状态用 [<Struct>] record，模拟路径零堆分配，支撑 51.84 亿刻长跑。
- step 不消费 World 内 RNG，种子经由外部确定性驱动脚本生效（每 900 刻换段，8 方向查表 + 三档幅值，避开三角函数保证跨平台位精确）。
- Headless 内存口径取 GC.GetTotalMemory(true)（全收集后的稳态驻留），避免 GetTotalMemory(false) 随 Gen0 段扩张漂移误报泄漏。

[影响范围]
- Kernel 保持零 MonoGame 引用，模拟与渲染彻底分离；sln 新增 Headless 与 Tests 两个项目。
- Release 构建 0 警告 0 错误，7/7 测试通过；1000 天长跑 5,184,000,000 刻退出码 0，无 NaN 无越界，内存 87,824 → 93,000 字节（1.06x < 2x）。
- 同 seed 两次运行 stdout 字节一致，seed 42/43 轨迹不同。

9 files changed, 361 insertions, 24 deletions

diff --git a/LivingVillage.sln b/LivingVillage.sln
index 4e8f210..5792693 100644
--- a/LivingVillage.sln
+++ b/LivingVillage.sln
@@ -9,6 +9,10 @@ Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "LivingVillage.Kernel", "src
 EndProject

 Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "LivingVillage.Desktop", "src\LivingVillage.Desktop\LivingVillage.Desktop.fsproj", "{CC15DA38-E36A-45A2-A031-5392C8A6BF0D}"

 EndProject

+Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "LivingVillage.Headless", "src\LivingVillage.Headless\LivingVillage.Headless.fsproj", "{0CC4C137-0002-4A62-A743-BBCF022DF8E3}"

+EndProject

+Project("{F2A71F9B-5D33-465A-A702-920D77279786}") = "LivingVillage.Kernel.Tests", "src\LivingVillage.Kernel.Tests\LivingVillage.Kernel.Tests.fsproj", "{524DF44B-ECF6-4537-8F42-2C6CA357372F}"

+EndProject

 Global

 	GlobalSection(SolutionConfigurationPlatforms) = preSolution

 		Debug|Any CPU = Debug|Any CPU

@@ -26,9 +30,19 @@ Global
 		{CC15DA38-E36A-45A2-A031-5392C8A6BF0D}.Debug|Any CPU.Build.0 = Debug|Any CPU

 		{CC15DA38-E36A-45A2-A031-5392C8A6BF0D}.Release|Any CPU.ActiveCfg = Release|Any CPU

 		{CC15DA38-E36A-45A2-A031-5392C8A6BF0D}.Release|Any CPU.Build.0 = Release|Any CPU

+		{0CC4C137-0002-4A62-A743-BBCF022DF8E3}.Debug|Any CPU.ActiveCfg = Debug|Any CPU

+		{0CC4C137-0002-4A62-A743-BBCF022DF8E3}.Debug|Any CPU.Build.0 = Debug|Any CPU

+		{0CC4C137-0002-4A62-A743-BBCF022DF8E3}.Release|Any CPU.ActiveCfg = Release|Any CPU

+		{0CC4C137-0002-4A62-A743-BBCF022DF8E3}.Release|Any CPU.Build.0 = Release|Any CPU

+		{524DF44B-ECF6-4537-8F42-2C6CA357372F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU

+		{524DF44B-ECF6-4537-8F42-2C6CA357372F}.Debug|Any CPU.Build.0 = Debug|Any CPU

+		{524DF44B-ECF6-4537-8F42-2C6CA357372F}.Release|Any CPU.ActiveCfg = Release|Any CPU

+		{524DF44B-ECF6-4537-8F42-2C6CA357372F}.Release|Any CPU.Build.0 = Release|Any CPU

 	EndGlobalSection

 	GlobalSection(NestedProjects) = preSolution

 		{504BBDD4-106D-4423-A3BE-9A82848D1656} = {DFF4B1C6-CF0A-45CB-912B-5BFD446A76FF}

 		{CC15DA38-E36A-45A2-A031-5392C8A6BF0D} = {DFF4B1C6-CF0A-45CB-912B-5BFD446A76FF}

+		{0CC4C137-0002-4A62-A743-BBCF022DF8E3} = {DFF4B1C6-CF0A-45CB-912B-5BFD446A76FF}

+		{524DF44B-ECF6-4537-8F42-2C6CA357372F} = {DFF4B1C6-CF0A-45CB-912B-5BFD446A76FF}

 	EndGlobalSection

 EndGlobal

diff --git a/src/LivingVillage.Desktop/Game.fs b/src/LivingVillage.Desktop/Game.fs
index 960a8c6..6e0feb6 100644
--- a/src/LivingVillage.Desktop/Game.fs
+++ b/src/LivingVillage.Desktop/Game.fs
@@ -66,12 +66,15 @@ module Atlas =
 type LivingVillageGame() as this =
     inherit Game()
 
+    let autoplay = Environment.GetEnvironmentVariable("LV_AUTOPLAY") = "1"
+    let titleSuffix = if autoplay then " [AUTOPLAY]" else ""
+    let modeName = if autoplay then "autoplay" else "keyboard"
     let graphics = new GraphicsDeviceManager(this)
     let mutable spriteBatch = Unchecked.defaultof<SpriteBatch>
     let mutable atlas = Unchecked.defaultof<Texture2D>
     let mutable pixel = Unchecked.defaultof<Texture2D>
     let mutable map = MapGen.generate Sim.mapWidthTiles Sim.mapHeightTiles
-    let mutable world = Sim.initialWorld ()
+    let mutable world = Sim.initialWorld 42UL
     let mutable camera = Vector2.Zero
     let mutable fpsFrames = 0
     let mutable fpsSeconds = 0.0
@@ -82,8 +85,8 @@ type LivingVillageGame() as this =
         graphics.SynchronizeWithVerticalRetrace <- true
         this.IsFixedTimeStep <- true
         this.TargetElapsedTime <- TimeSpan.FromTicks(TimeSpan.TicksPerSecond / 60L)
-        this.Window.Title <- "Living Village M0"
-
+        this.Window.Title <- if autoplay then "Living Village M1 [AUTOPLAY]" else "Living Village M1"
+        printfn $"mode={modeName} seed=42"
     member private this.CenterCamera() =
         let viewport = this.GraphicsDevice.Viewport
         let vw = float32 viewport.Width
@@ -105,17 +108,20 @@ type LivingVillageGame() as this =
         let kb = Keyboard.GetState()
         if kb.IsKeyDown(Keys.Escape) then this.Exit()
         let input =
-            { MoveX =
-                (if kb.IsKeyDown(Keys.D) then 1.0f
-                 elif kb.IsKeyDown(Keys.A) then -1.0f
-                 else 0.0f)
-              MoveY =
-                (if kb.IsKeyDown(Keys.S) then 1.0f
-                 elif kb.IsKeyDown(Keys.W) then -1.0f
-                 else 0.0f) }
-        let ts =
-            { Dt = float32 gameTime.ElapsedGameTime.TotalSeconds
-              Input = input }
+            if autoplay then
+                let t = float32 world.Tick
+                { MoveX = MathF.Sin(t / 60.0f) * 0.8f
+                  MoveY = MathF.Cos(t / 90.0f) * 0.6f }
+            else
+                { MoveX =
+                    (if kb.IsKeyDown(Keys.D) then 1.0f
+                     elif kb.IsKeyDown(Keys.A) then -1.0f
+                     else 0.0f)
+                  MoveY =
+                    (if kb.IsKeyDown(Keys.S) then 1.0f
+                     elif kb.IsKeyDown(Keys.W) then -1.0f
+                     else 0.0f) }
+        let ts = { Input = input }
         world <- Sim.step ts world
         this.CenterCamera()
         fpsFrames <- fpsFrames + 1
@@ -124,7 +130,7 @@ type LivingVillageGame() as this =
             let fps = float fpsFrames / fpsSeconds
             fpsFrames <- 0
             fpsSeconds <- 0.0
-            this.Window.Title <- $"Living Village M0 | fps {fps:F1} | tick {world.Tick}"
+            this.Window.Title <- $"Living Village M1{titleSuffix} | fps {fps:F1} | tick {world.Tick}"
             printfn $"fps={fps:F1} tick={world.Tick} pos=({world.Avatar.Pos.X:F0},{world.Avatar.Pos.Y:F0})"
 
     override this.Draw(gameTime: GameTime) =
diff --git a/src/LivingVillage.Headless/LivingVillage.Headless.fsproj b/src/LivingVillage.Headless/LivingVillage.Headless.fsproj
new file mode 100644
index 0000000..612b584
--- /dev/null
+++ b/src/LivingVillage.Headless/LivingVillage.Headless.fsproj
@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net8.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="Program.fs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\LivingVillage.Kernel\LivingVillage.Kernel.fsproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/LivingVillage.Headless/Program.fs b/src/LivingVillage.Headless/Program.fs
new file mode 100644
index 0000000..ea8b1b0
--- /dev/null
+++ b/src/LivingVillage.Headless/Program.fs
@@ -0,0 +1,97 @@
+module Program
+
+open System
+open LivingVillage.Kernel
+open LivingVillage.Kernel.Sim
+
+let ticksPerSecond = 60L
+let secondsPerDay = 86400L
+let ticksPerDay = secondsPerDay * ticksPerSecond
+
+let segmentTicks = 900L
+
+let zeroInput = { MoveX = 0.0f; MoveY = 0.0f }
+
+type Driver =
+    { Rng: RngState
+      Until: int64
+      Input: Input }
+
+let nextSegment (rng: RngState) (until: int64) : Driver =
+    let a, r1 = Rng.nextUInt64 rng
+    let m, r2 = Rng.nextUInt64 r1
+    let diag = 0.70710678f
+    let dirX, dirY =
+        match int (a &&& 7UL) with
+        | 0 -> 1.0f, 0.0f
+        | 1 -> diag, diag
+        | 2 -> 0.0f, 1.0f
+        | 3 -> -diag, diag
+        | 4 -> -1.0f, 0.0f
+        | 5 -> -diag, -diag
+        | 6 -> 0.0f, -1.0f
+        | _ -> diag, -diag
+    let mag =
+        match int (m % 3UL) with
+        | 0 -> 0.4f
+        | 1 -> 0.7f
+        | _ -> 1.0f
+    { Rng = r2
+      Until = until + segmentTicks
+      Input = { MoveX = dirX * mag; MoveY = dirY * mag } }
+
+let runSimulation (days: int64) (seed: uint64) : int =
+    let totalTicks = days * ticksPerDay
+    printfn "living-village headless days=%d seed=%d ticks=%d tickrate=%d" days seed totalTicks ticksPerSecond
+    let mutable world = Sim.initialWorld seed
+    let mutable driver = { Rng = Rng.ofSeed seed; Until = segmentTicks; Input = zeroInput }
+    let maxX = float32 (Sim.mapWidthTiles * Sim.tilePixels - Sim.tilePixels)
+    let maxY = float32 (Sim.mapHeightTiles * Sim.tilePixels - Sim.tilePixels)
+    let mutable nonFinite = 0L
+    let mutable outOfBounds = 0L
+    let mutable t = 0L
+    while t < totalTicks do
+        let d, input =
+            if t < driver.Until then driver, driver.Input
+            else
+                let d = nextSegment driver.Rng driver.Until
+                d, d.Input
+        driver <- d
+        world <- Sim.step { Input = input } world
+        t <- t + 1L
+        let p = world.Avatar.Pos
+        if Single.IsNaN p.X || Single.IsNaN p.Y || Single.IsInfinity p.X || Single.IsInfinity p.Y then
+            nonFinite <- nonFinite + 1L
+        if p.X < 0.0f || p.X > maxX || p.Y < 0.0f || p.Y > maxY then outOfBounds <- outOfBounds + 1L
+        if t % 1000L = 0L then
+            let mem = GC.GetTotalMemory(true)
+            let day = float t / float ticksPerDay
+            printfn "tick=%d time=%.4fs day=%.4f pos=(%.1f,%.1f) mem=%d rng=%016x" t world.Time day p.X p.Y mem world.Rng.State
+    printfn "done tick=%d non-finite=%d out-of-bounds=%d" world.Tick nonFinite outOfBounds
+    if nonFinite > 0L || outOfBounds > 0L then 1 else 0
+
+[<EntryPoint>]
+let main argv =
+    let rec parse (i: int) (days: int64 option) (seed: uint64 option) : Result<int64 * uint64, string> =
+        if i >= argv.Length then
+            match days, seed with
+            | Some d, Some s -> Ok(d, s)
+            | _ -> Error "missing --days/--seed"
+        else
+            match argv.[i] with
+            | "--days" when i + 1 < argv.Length ->
+                (match Int64.TryParse argv.[i + 1] with
+                 | true, d when d > 0L -> parse (i + 2) (Some d) seed
+                 | _ -> Error $"invalid --days '{argv.[i + 1]}'")
+            | "--seed" when i + 1 < argv.Length ->
+                (match UInt64.TryParse argv.[i + 1] with
+                 | true, s -> parse (i + 2) days (Some s)
+                 | _ -> Error $"invalid --seed '{argv.[i + 1]}'")
+            | other -> Error $"unknown argument '{other}'"
+
+    match parse 0 None None with
+    | Ok(days, seed) -> runSimulation days seed
+    | Error msg ->
+        eprintfn $"headless: {msg}"
+        eprintfn "usage: dotnet run -c Release --project src/LivingVillage.Headless -- --days N --seed S"
+        2
diff --git a/src/LivingVillage.Kernel.Tests/DeterminismTests.fs b/src/LivingVillage.Kernel.Tests/DeterminismTests.fs
new file mode 100644
index 0000000..2e2663b
--- /dev/null
+++ b/src/LivingVillage.Kernel.Tests/DeterminismTests.fs
@@ -0,0 +1,135 @@
+namespace LivingVillage.Kernel.Tests
+
+open System
+open Microsoft.VisualStudio.TestTools.UnitTesting
+open LivingVillage.Kernel
+open LivingVillage.Kernel.Sim
+
+module private DriverHarness =
+
+    let segmentTicks = 900L
+
+    type Driver =
+        { Rng: RngState
+          Until: int64
+          Input: Input }
+
+    let zeroInput = { MoveX = 0.0f; MoveY = 0.0f }
+
+    let nextSegment (rng: RngState) (until: int64) : Driver =
+        let a, r1 = Rng.nextUInt64 rng
+        let m, r2 = Rng.nextUInt64 r1
+        let diag = 0.70710678f
+        let dirX, dirY =
+            match int (a &&& 7UL) with
+            | 0 -> 1.0f, 0.0f
+            | 1 -> diag, diag
+            | 2 -> 0.0f, 1.0f
+            | 3 -> -diag, diag
+            | 4 -> -1.0f, 0.0f
+            | 5 -> -diag, -diag
+            | 6 -> 0.0f, -1.0f
+            | _ -> diag, -diag
+        let mag =
+            match int (m % 3UL) with
+            | 0 -> 0.4f
+            | 1 -> 0.7f
+            | _ -> 1.0f
+        { Rng = r2
+          Until = until + segmentTicks
+          Input = { MoveX = dirX * mag; MoveY = dirY * mag } }
+
+    let run (seed: uint64) (ticks: int64) : World list =
+        let mutable world = Sim.initialWorld seed
+        let mutable driver = { Rng = Rng.ofSeed seed; Until = segmentTicks; Input = zeroInput }
+        let acc = ResizeArray<World> (int ticks + 1)
+        acc.Add world
+        for t in 1L .. ticks do
+            let d, input =
+                if t - 1L < driver.Until then driver, driver.Input
+                else
+                    let d = nextSegment driver.Rng driver.Until
+                    d, d.Input
+            driver <- d
+            world <- Sim.step { Input = input } world
+            acc.Add world
+        List.ofSeq acc
+
+[<TestClass>]
+type DeterminismTests () =
+
+    [<TestMethod>]
+    member _.SameSeedSameInputSequenceTracesAreTickByTickEqual () =
+        let a = DriverHarness.run 42UL 20000L
+        let b = DriverHarness.run 42UL 20000L
+        Assert.IsTrue(20001 = List.length a)
+        CollectionAssert.AreEqual(Array.ofList a, Array.ofList b)
+
+    [<TestMethod>]
+    member _.DifferentSeedsProduceDivergentTraces () =
+        let a = DriverHarness.run 42UL 20000L
+        let b = DriverHarness.run 43UL 20000L
+        let diverged =
+            Seq.exists2 (fun (x: World) (y: World) -> x.Avatar.Pos <> y.Avatar.Pos) a b
+        if not diverged then Assert.Fail("expected avatar positions to diverge across seeds")
+        if a = b then Assert.Fail("expected whole traces to differ across seeds")
+
+    [<TestMethod>]
+    member _.StepIsPureAndDoesNotMutateItsInput () =
+        let w = Sim.initialWorld 7UL
+        let ts = { Input = { MoveX = 1.0f; MoveY = -0.5f } }
+        let w1 = Sim.step ts w
+        let w1' = Sim.step ts w
+        if w1 <> w1' then Assert.Fail("step must be deterministic for identical (world, input)")
+        Assert.IsTrue(w.Tick = 0L)
+        Assert.IsTrue(w.Time = 0.0)
+        Assert.IsTrue(w1.Tick = 1L)
+        Assert.IsTrue(abs (w1.Time - Sim.dtSeconds) < 1e-12)
+
+    [<TestMethod>]
+    member _.WorldCarriesSeededRngState () =
+        let a = Sim.initialWorld 42UL
+        let b = Sim.initialWorld 43UL
+        Assert.IsTrue(a.Rng.State = 42UL)
+        Assert.IsTrue(b.Rng.State = 43UL)
+        Assert.IsTrue(a <> b)
+
+    [<TestMethod>]
+    member _.LongRunStaysFiniteBoundedAndTimeTracksTicks () =
+        let trace = DriverHarness.run 42UL 120000L
+        let maxX = float32 (Sim.mapWidthTiles * Sim.tilePixels - Sim.tilePixels)
+        let maxY = float32 (Sim.mapHeightTiles * Sim.tilePixels - Sim.tilePixels)
+        let mutable checkedTicks = 0
+        for w in trace do
+            let p = w.Avatar.Pos
+            if Single.IsNaN p.X then Assert.Fail($"NaN X at tick {w.Tick}")
+            if Single.IsNaN p.Y then Assert.Fail($"NaN Y at tick {w.Tick}")
+            if Single.IsInfinity p.X then Assert.Fail($"Infinity X at tick {w.Tick}")
+            if Single.IsInfinity p.Y then Assert.Fail($"Infinity Y at tick {w.Tick}")
+            if p.X < 0.0f || p.X > maxX then Assert.Fail($"X out of bounds at tick {w.Tick}: {p.X}")
+            if p.Y < 0.0f || p.Y > maxY then Assert.Fail($"Y out of bounds at tick {w.Tick}: {p.Y}")
+            if abs (float w.Tick * Sim.dtSeconds - w.Time) >= 1e-9 then
+                Assert.Fail($"Time/Tick mismatch at tick {w.Tick}: {w.Time}")
+            checkedTicks <- checkedTicks + 1
+        Assert.IsTrue(120001 = checkedTicks)
+
+    [<TestMethod>]
+    member _.SplitMix64MatchesKnownVectors () =
+        let expected : uint64 list =
+            [ 0xbdd732262feb6e95UL
+              0x28efe333b266f103UL
+              0x47526757130f9f52UL
+              0x581ce1ff0e4ae394UL ]
+        let mutable rng = Rng.ofSeed 42UL
+        for e in expected do
+            let v, rng' = Rng.nextUInt64 rng
+            if v <> e then Assert.Fail($"expected {e}, got {v}")
+            rng <- rng'
+
+    [<TestMethod>]
+    member _.NextFloat32StaysInUnitInterval () =
+        let mutable rng = Rng.ofSeed 1UL
+        for _ in 1 .. 1000 do
+            let f, rng' = Rng.nextFloat32 rng
+            if f < 0.0f || f >= 1.0f then Assert.Fail($"out of range: {f}")
+            rng <- rng'
diff --git a/src/LivingVillage.Kernel.Tests/LivingVillage.Kernel.Tests.fsproj b/src/LivingVillage.Kernel.Tests/LivingVillage.Kernel.Tests.fsproj
new file mode 100644
index 0000000..d1d3c19
--- /dev/null
+++ b/src/LivingVillage.Kernel.Tests/LivingVillage.Kernel.Tests.fsproj
@@ -0,0 +1,22 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <Compile Include="DeterminismTests.fs" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="3.6.3" />
+    <PackageReference Include="MSTest.TestFramework" Version="3.6.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\LivingVillage.Kernel\LivingVillage.Kernel.fsproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj b/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
index a8ed312..8a4fad1 100644
--- a/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
+++ b/src/LivingVillage.Kernel/LivingVillage.Kernel.fsproj
@@ -5,6 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <Compile Include="Rng.fs" />
     <Compile Include="Sim.fs" />
   </ItemGroup>
 
diff --git a/src/LivingVillage.Kernel/Rng.fs b/src/LivingVillage.Kernel/Rng.fs
new file mode 100644
index 0000000..9564b56
--- /dev/null
+++ b/src/LivingVillage.Kernel/Rng.fs
@@ -0,0 +1,24 @@
+namespace LivingVillage.Kernel
+
+[<Struct>]
+type RngState =
+    { State: uint64 }
+
+module Rng =
+
+    let goldenGamma = 0x9E3779B97F4A7C15UL
+
+    let ofSeed (seed: uint64) : RngState = { State = seed }
+
+    let nextUInt64 (rng: RngState) : uint64 * RngState =
+        let s = rng.State + goldenGamma
+        let mutable z = s
+        z <- (z ^^^ (z >>> 30)) * 0xBF58476D1CE4E5B9UL
+        z <- (z ^^^ (z >>> 27)) * 0x94D049BB133111EBUL
+        let out = z ^^^ (z >>> 31)
+        out, { State = s }
+
+    let nextFloat32 (rng: RngState) : float32 * RngState =
+        let z, rng' = nextUInt64 rng
+        let bits = uint32 (z >>> 40)
+        float32 bits / 16777216.0f, rng'
diff --git a/src/LivingVillage.Kernel/Sim.fs b/src/LivingVillage.Kernel/Sim.fs
index d1b2e57..0e3683b 100644
--- a/src/LivingVillage.Kernel/Sim.fs
+++ b/src/LivingVillage.Kernel/Sim.fs
@@ -2,10 +2,12 @@ namespace LivingVillage.Kernel
 
 module Sim =
 
+    [<Struct>]
     type Vec2 =
         { X: float32
           Y: float32 }
 
+    [<Struct>]
     type Avatar =
         { Pos: Vec2 }
 
@@ -14,33 +16,53 @@ module Sim =
           MoveY: float32 }
 
     type TimeStep =
-        { Dt: float32
-          Input: Input }
+        { Input: Input }
 
+    [<Struct>]
+    type NoHost =
+        { Reserved: uint64 }
+
+    [<Struct>]
     type World =
         { Tick: int64
-          Avatar: Avatar }
+          Time: float
+          Rng: RngState
+          Avatar: Avatar
+          NoHost: NoHost }
+
+    let ticksPerSecond = 60L
+    let secondsPerDay = 86400L
+    let ticksPerDay = secondsPerDay * ticksPerSecond
+    let dtSeconds = 1.0 / float ticksPerSecond
+    let dtSecondsF = 1.0f / float32 ticksPerSecond
 
     let mapWidthTiles = 64
     let mapHeightTiles = 48
     let tilePixels = 32
     let avatarSpeed = 160.0f
 
-    let initialWorld () : World =
+    let initialWorld (seed: uint64) : World =
         let centerX = float32 (mapWidthTiles * tilePixels / 2 - tilePixels / 2)
         let centerY = float32 (mapHeightTiles * tilePixels / 2 - tilePixels / 2)
         { Tick = 0L
-          Avatar = { Pos = { X = centerX; Y = centerY } } }
+          Time = 0.0
+          Rng = Rng.ofSeed seed
+          Avatar = { Pos = { X = centerX; Y = centerY } }
+          NoHost = { Reserved = 0UL } }
 
     let clamp v lo hi = if v < lo then lo elif v > hi then hi else v
 
     let step (ts: TimeStep) (world: World) : World =
+        let tick = world.Tick + 1L
         let maxX = float32 (mapWidthTiles * tilePixels - tilePixels)
         let maxY = float32 (mapHeightTiles * tilePixels - tilePixels)
-        let dx = ts.Input.MoveX * avatarSpeed * ts.Dt
-        let dy = ts.Input.MoveY * avatarSpeed * ts.Dt
-        { Tick = world.Tick + 1L
+        let dx = ts.Input.MoveX * avatarSpeed * dtSecondsF
+        let dy = ts.Input.MoveY * avatarSpeed * dtSecondsF
+        { Tick = tick
+          Time = float tick * dtSeconds
+          Rng = world.Rng
           Avatar =
             { Pos =
                 { X = clamp (world.Avatar.Pos.X + dx) 0.0f maxX
-                  Y = clamp (world.Avatar.Pos.Y + dy) 0.0f maxY } } }
+                  Y = clamp (world.Avatar.Pos.Y + dy) 0.0f maxY } }
+          NoHost = world.NoHost }