feat(mapgen): 生成器序列化 + 世界站位接入（P20 第三步 A）

- MapGen.serialize：生成结果的确定性序列化（尺寸/seed/瓦片/Rivers/Bridges/
  Paths/Buildings），同 seed 逐字节一致，供回归与跨机核对。
- WorldBootstrap.initialWorldWithPlacement：显式站位建世界；旧 64x48 路径
  （initialWorld/initialWorldWithOccupation）字节口径不变。
- WorldBootstrap.initialWorldInMap：用生成器核心可达区做站位，30 spawn 全落
  可达核心；新增 serialization/placement 两条测试。
- fsproj：MapGen 先于 WorldBootstrap 编译。
- Desktop 119->121，Kernel 90 不变。

1 files changed, 25 insertions, 0 deletions

diff --git a/src/LivingVillage.Desktop.Tests/MapGenTests.fs b/src/LivingVillage.Desktop.Tests/MapGenTests.fs
index b0b58e7..4079ad0 100644
--- a/src/LivingVillage.Desktop.Tests/MapGenTests.fs
+++ b/src/LivingVillage.Desktop.Tests/MapGenTests.fs
@@ -147,6 +147,31 @@ type MapGenTests () =
         let c = MapGen.generateWithSize 256 192 (uint64 7)
         Assert.IsTrue(a.Buildings <> c.Buildings || a.Bridges <> c.Bridges, "different seeds must relocate structures")
 
+    // ---- P20 第三步：序列化确定性 + 默认可玩世界接入生成器站位 ----
+
+    [<TestMethod>]
+    member _.SameSeedSerializationIsByteIdentical () =
+        let a = MapGen.generateWithSize 256 192 (uint64 4242)
+        let b = MapGen.generateWithSize 256 192 (uint64 4242)
+        Assert.AreEqual<string>(MapGen.serialize a, MapGen.serialize b)
+        let c = MapGen.generateWithSize 256 192 (uint64 7)
+        Assert.AreNotEqual<string>(MapGen.serialize a, MapGen.serialize c)
+
+    [<TestMethod>]
+    member _.MapWorldPlacementUsesReachableCoreSpawns () =
+        let map = MapGen.generateWithSize 256 192 (uint64 4242)
+        let world = WorldBootstrap.initialWorldInMap false 42UL Sim.npcCount map
+        Assert.AreEqual<int>(Sim.npcCount, world.Npcs.Length)
+        let visited = MapGen.floodFill map (fst map.Spawns.Head) (snd map.Spawns.Head)
+        for npc in world.Npcs do
+            let tx = int npc.Pos.X / Sim.tilePixels
+            let ty = int npc.Pos.Y / Sim.tilePixels
+            Assert.IsTrue(visited.[ty * map.Width + tx], sprintf "npc tile (%d,%d) must be reachable from the core" tx ty)
+        let ax = int world.Avatar.Pos.X / Sim.tilePixels
+        let ay = int world.Avatar.Pos.Y / Sim.tilePixels
+        Assert.IsTrue(visited.[ay * map.Width + ax], "avatar spawn must sit in the reachable core")
+        Assert.IsTrue(ax >= map.Core.MinX && ax <= map.Core.MaxX && ay >= map.Core.MinY && ay <= map.Core.MaxY, "avatar spawn must sit in the core")
+
     [<TestMethod>]
     member _.VisibleRangeClampsAtLegacyMapEdges () =
         let x0, y0, x1, y1 = MapGen.visibleTileRange 64 48 0 0 2048 1536