feat(desktop): 村民与主角四帧行走动画

人物图集 5 变体 x 4 向 x 4 帧（2560x48，全部经 make-jiangnan-art.py 原创，四种腿步：站立/左跨/站立微跳/右跨镜像）；animationFrame 按 tick/4 mod 4 确定性取帧（16 tick 一循环，禁实时时钟）；快照式回归断言四帧循环与逐帧图集列映射，主角同规则。

3 files changed, 38 insertions, 6 deletions

diff --git a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
index d77abd6..450e94b 100644
--- a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
+++ b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
@@ -324,7 +324,7 @@ type PrototypeTests () =
         Assert.IsTrue(first.PanelY + first.PanelHeight <= 720)
 
     [<TestMethod>]
-    member _.NpcVariantsAreStableAndAnimationCoversFourDirectionsAndTwoFrames () =
+    member _.NpcVariantsAreStableAndAnimationCoversFourDirectionsAndFourFrames () =
         let sameNpc = npcVisualVariant (NpcId 7)
         let repeatedNpc = npcVisualVariant (NpcId 7)
         let otherNpc = npcVisualVariant (NpcId 8)
@@ -341,7 +341,9 @@ type PrototypeTests () =
         Assert.IsTrue(sprites |> List.forall (fun sprite -> sprite.Frame = FrameTwo))
         Assert.AreEqual<AnimationFrame>(FrameOne, (VillageArt.animationFrame 0L))
         Assert.AreEqual<AnimationFrame>(FrameTwo, (VillageArt.animationFrame 4L))
-        Assert.AreEqual<AnimationFrame>(FrameOne, (VillageArt.animationFrame 8L))
+        Assert.AreEqual<AnimationFrame>(FrameThree, (VillageArt.animationFrame 8L))
+        Assert.AreEqual<AnimationFrame>(FrameFour, (VillageArt.animationFrame 12L))
+        Assert.AreEqual<AnimationFrame>(FrameOne, (VillageArt.animationFrame 16L))
 
     [<TestMethod>]
     member _.VillagersUseFourDistinctVariantsDistinctFromTheAvatar () =
@@ -359,7 +361,28 @@ type PrototypeTests () =
                 rect.X)
             |> Set.ofList
         Assert.AreEqual<int>(4, variantXes.Count)
-        Assert.AreEqual<int>(1280, VillageArt.characterAtlasWidth)
+        Assert.AreEqual<int>(2560, VillageArt.characterAtlasWidth)
+
+    [<TestMethod>]
+    member _.WalkAnimationCoversFourFramesDeterministically () =
+        // 16-tick cycle, new frame every 4 sim ticks, no wall-clock dependency.
+        let frames = [ 0L; 4L; 8L; 12L; 16L; 20L ] |> List.map VillageArt.animationFrame
+        Assert.AreEqual<AnimationFrame list>([ FrameOne; FrameTwo; FrameThree; FrameFour; FrameOne; FrameTwo ], frames)
+
+        let cycleFrames =
+            [ 0L; 4L; 8L; 12L ]
+            |> List.map VillageArt.animationFrame
+            |> List.distinct
+        Assert.AreEqual<AnimationFrame list>([ FrameOne; FrameTwo; FrameThree; FrameFour ], cycleFrames)
+
+        // Every frame maps to a distinct atlas cell column across the five variants.
+        // Every frame maps to a distinct atlas cell column across the five variants.
+        let perFrameX =
+            [ FrameOne; FrameTwo; FrameThree; FrameFour ]
+            |> List.map (fun frame ->
+                let rect = characterSourceRectangle { Variant = IndigoRobe; Direction = NorthFacing; Frame = frame }
+                rect.X / 32)
+        Assert.AreEqual<int list>([ 0; 1; 2; 3 ], perFrameX)
 
     [<TestMethod>]
     member _.ChineseCopyProvidesFormalSceneTimeMenuAndStatusLabels () =
diff --git a/src/LivingVillage.Desktop/Assets/jiangnan-characters.png b/src/LivingVillage.Desktop/Assets/jiangnan-characters.png
index cbbe570..f6c8711 100644
--- a/src/LivingVillage.Desktop/Assets/jiangnan-characters.png
+++ b/src/LivingVillage.Desktop/Assets/jiangnan-characters.png
diff --git a/src/LivingVillage.Desktop/VillageArt.fs b/src/LivingVillage.Desktop/VillageArt.fs
index db25757..6f5af07 100644
--- a/src/LivingVillage.Desktop/VillageArt.fs
+++ b/src/LivingVillage.Desktop/VillageArt.fs
@@ -84,6 +84,8 @@ module VillageArt =
     type AnimationFrame =
         | FrameOne
         | FrameTwo
+        | FrameThree
+        | FrameFour
 
     type NpcSpriteSpec =
         { Variant: NpcVisualVariant
@@ -250,8 +252,13 @@ module VillageArt =
         let remainder = value % divisor
         if remainder < 0L then remainder + divisor else remainder
 
+    /// Four-pose walk cycle over a 16-tick window (one pose per 4 sim ticks).
     let animationFrame (tick: int64) : AnimationFrame =
-        if positiveModulo64 8L tick < 4L then FrameOne else FrameTwo
+        match positiveModulo64 4L (tick / 4L) with
+        | 0L -> FrameOne
+        | 1L -> FrameTwo
+        | 2L -> FrameThree
+        | _ -> FrameFour
 
     /// Water surface ripple phase: flips every 32 simulation ticks, fully deterministic.
     let waterFrameTick (tick: int64) : int =
@@ -303,7 +310,9 @@ module VillageArt =
             match spec.Frame with
             | FrameOne -> 0
             | FrameTwo -> 1
-        let cellIndex = (variantIndex * 4 + directionIndex) * 2 + frameIndex
+            | FrameThree -> 2
+            | FrameFour -> 3
+        let cellIndex = (variantIndex * 4 + directionIndex) * 4 + frameIndex
         Microsoft.Xna.Framework.Rectangle(cellIndex * 32, 0, 32, 48)
 
     let private rgba (r: int) (g: int) (b: int) (a: int) : XnaColor = XnaColor(r, g, b, a)
@@ -320,7 +329,7 @@ module VillageArt =
     let worldTileCount = 21
     let worldAtlasWidth = Sim.tilePixels * worldTileCount
     let worldAtlasHeight = Sim.tilePixels
-    let characterAtlasWidth = 32 * 5 * 4 * 2
+    let characterAtlasWidth = 32 * 5 * 4 * 4
     let characterAtlasHeight = 48
     let interiorAtlasWidth = 392
     let interiorAtlasHeight = 96