feat(desktop): 江南水乡河道横贯与波纹两帧

场景改为沿村蜿蜒水带（北岸凸湾、中段横贯、南段收窄），南北两岸石板路沿河展开，木桥三块纵跨全河连接民居门前石径；水面新增第二张波纹图块，按世界 tick/32 取模确定性交替，绘制无新增每帧分配。

4 files changed, 69 insertions, 17 deletions

diff --git a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
index 06b3250..7959d27 100644
--- a/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
+++ b/src/LivingVillage.Desktop.Tests/PrototypeTests.fs
@@ -301,8 +301,42 @@ type PrototypeTests () =
             Assert.IsTrue(footprint.Contains({ X = door.X + dx; Y = door.Y - 2 } : TilePosition), sprintf "missing ridge tile dx=%d" dx)
             Assert.IsTrue(footprint.Contains({ X = door.X + dx; Y = door.Y - 1 } : TilePosition), sprintf "missing eave tile dx=%d" dx)
             Assert.IsTrue(footprint.Contains({ X = door.X + dx; Y = door.Y } : TilePosition), sprintf "missing wall tile dx=%d" dx)
-        // 12 house tiles + 2 bridge + 3 bamboo + 2 garden.
-        Assert.AreEqual<int>(19, footprint.Count)
+        // 12 house tiles + 3 bridge + 3 bamboo + 2 garden.
+        Assert.AreEqual<int>(20, footprint.Count)
+
+    [<TestMethod>]
+    member _.WaterRippleFramesAreDeterministicAndTwoPhase () =
+        Assert.AreEqual<int>(0, VillageArt.waterFrameTick 0L)
+        Assert.AreEqual<int>(0, VillageArt.waterFrameTick 31L)
+        Assert.AreEqual<int>(1, VillageArt.waterFrameTick 32L)
+        Assert.AreEqual<int>(1, VillageArt.waterFrameTick 63L)
+        Assert.AreEqual<int>(0, VillageArt.waterFrameTick 64L)
+        Assert.AreEqual<int>(VillageArt.waterFrameTick 100L, VillageArt.waterFrameTick 100L)
+
+    [<TestMethod>]
+    member _.RiverBandSpansTheSceneWithBankPathsAndCrossing () =
+        let plan = VillageArt.sampleRenderPlan ()
+        let footprintOf (value: VillageArt.JiangnanRenderPlan) = VillageArt.planFootprint value
+        let waters = plan.Grounds |> List.filter (fun (_, kind) -> kind = VillageArt.Water) |> List.map fst |> Set.ofList
+        let paths = plan.Grounds |> List.filter (fun (_, kind) -> kind = VillageArt.StonePath) |> List.map fst |> Set.ofList
+        let door = scene.Homes |> List.head |> fun home -> home.Door
+
+        // The river must run the full width of the decorated scene as a readable water band.
+        Assert.AreEqual<int>(20, [ 0 .. 19 ] |> List.filter (fun x -> waters.Contains({ X = x; Y = 12 } : TilePosition)) |> List.length)
+        Assert.IsTrue(waters.Contains({ X = 6; Y = 13 } : TilePosition))
+        // Banks stay walkable stone, and the door row connects to the north bank.
+        for x in 0 .. 19 do
+            if x <> 6 && x <> 7 && x <> 8 then
+                Assert.IsTrue(paths.Contains({ X = x; Y = 11 } : TilePosition), sprintf "missing north bank x=%d" x)
+        for y in 6 .. 10 do
+            Assert.IsTrue(paths.Contains({ X = 6; Y = y } : TilePosition), sprintf "missing door connector y=%d" y)
+        // Bridge footprint crosses the full river between the banks.
+        let bridge = plan.Props |> List.find (fun prop -> prop.Kind = VillageArt.Bridge)
+        let expectedBridge : TilePosition = { X = 6; Y = 11 }
+        Assert.AreEqual<TilePosition>(expectedBridge, bridge.Position)
+        for y in 11 .. 13 do
+            let bridgeTile = VillageArt.worldTile plan { X = 6; Y = y }
+            Assert.IsTrue(footprintOf plan |> List.contains bridgeTile, sprintf "bridge tile y=%d" y)
 
     [<TestMethod>]
     member _.FacingUsesTheActualMovementOrTargetVector () =
diff --git a/src/LivingVillage.Desktop/Assets/jiangnan-world.png b/src/LivingVillage.Desktop/Assets/jiangnan-world.png
index 766405f..ca4a521 100644
--- a/src/LivingVillage.Desktop/Assets/jiangnan-world.png
+++ b/src/LivingVillage.Desktop/Assets/jiangnan-world.png
diff --git a/src/LivingVillage.Desktop/Game.fs b/src/LivingVillage.Desktop/Game.fs
index a5049f3..4871e80 100644
--- a/src/LivingVillage.Desktop/Game.fs
+++ b/src/LivingVillage.Desktop/Game.fs
@@ -576,6 +576,7 @@ type LivingVillageGame() as this =
                 viewport.Width
                 viewport.Height
                 renderPlan
+                world.Tick
                 profile.WorldTint
 
             let avatarSpec = VillageArt.avatarSpriteSpec avatarFacing world.Tick
diff --git a/src/LivingVillage.Desktop/VillageArt.fs b/src/LivingVillage.Desktop/VillageArt.fs
index 5883a97..a66474b 100644
--- a/src/LivingVillage.Desktop/VillageArt.fs
+++ b/src/LivingVillage.Desktop/VillageArt.fs
@@ -102,6 +102,7 @@ module VillageArt =
         | WallWoodSprite
         | WallWindowSprite
         | DoorSprite
+        | WaterSpriteB
 
     type private XnaColor = Microsoft.Xna.Framework.Color
     type private XnaRectangle = Microsoft.Xna.Framework.Rectangle
@@ -143,15 +144,7 @@ module VillageArt =
 
         { Origin = position 25 18
           Grounds =
-              [ ground Water 10 2
-                ground Water 10 3
-                ground Water 10 4
-                ground Water 10 5
-                ground Water 10 6
-                ground Water 10 7
-                ground Water 11 4
-                ground Water 11 5
-                ground StonePath 3 5
+              [ ground StonePath 3 5
                 ground StonePath 4 5
                 ground StonePath 5 5
                 ground StonePath 6 5
@@ -161,10 +154,26 @@ module VillageArt =
                 ground StonePath 10 5
                 ground StonePath 11 5
                 ground StonePath 6 6
-                ground StonePath 6 7 ]
+                ground StonePath 6 7
+                ground StonePath 6 8
+                ground StonePath 6 9
+                ground StonePath 6 10
+                for x in 0 .. 5 do
+                    ground StonePath x 11
+                for x in 9 .. 19 do
+                    ground StonePath x 11
+                for x in 1 .. 18 do
+                    ground StonePath x 14
+                ground Water 6 11
+                ground Water 7 11
+                ground Water 8 11
+                for x in 0 .. 19 do
+                    ground Water x 12
+                for x in 2 .. 17 do
+                    ground Water x 13 ]
           Props =
               [ renderProp House 6 5
-                renderProp Bridge 10 5
+                renderProp Bridge 6 11
                 renderProp BambooGrove 3 2
                 renderProp VegetablePatch 8 7 ]
           Interior =
@@ -190,7 +199,7 @@ module VillageArt =
                 [ for dx in -2 .. 1 do
                       for dy in -2 .. 0 do
                           yield position (x + dx) (y + dy) ]
-            | Bridge -> [ position x y; position x (y + 1) ]
+            | Bridge -> [ position x y; position x (y + 1); position x (y + 2) ]
             | BambooGrove -> [ position x y; position x (y + 1); position (x + 1) y ]
             | VegetablePatch -> [ position x y; position (x + 1) y ])
         |> many
@@ -212,6 +221,10 @@ module VillageArt =
     let animationFrame (tick: int64) : AnimationFrame =
         if positiveModulo64 8L tick < 4L then FrameOne else FrameTwo
 
+    /// Water surface ripple phase: flips every 32 simulation ticks, fully deterministic.
+    let waterFrameTick (tick: int64) : int =
+        int (positiveModulo64 2L (tick / 32L))
+
     let directionFromVector (vector: Vec2) : FacingDirection =
         if abs vector.X < 0.001f && abs vector.Y < 0.001f then
             SouthFacing
@@ -271,7 +284,7 @@ module VillageArt =
     let characterAtlasRelativePath = "Assets/jiangnan-characters.png"
     let interiorAtlasRelativePath = "Assets/jiangnan-interior.png"
 
-    let worldTileCount = 13
+    let worldTileCount = 14
     let worldAtlasWidth = Sim.tilePixels * worldTileCount
     let worldAtlasHeight = Sim.tilePixels
     let characterAtlasWidth = 32 * 4 * 4 * 2
@@ -313,6 +326,7 @@ module VillageArt =
             | WallWoodSprite -> 10
             | WallWindowSprite -> 11
             | DoorSprite -> 12
+            | WaterSpriteB -> 13
         XnaRectangle(index * Sim.tilePixels, 0, Sim.tilePixels, Sim.tilePixels)
 
     let private tilePosition x y : TilePosition =
@@ -350,9 +364,11 @@ module VillageArt =
         (viewportWidth: int)
         (viewportHeight: int)
         (plan: JiangnanRenderPlan)
+        (tick: int64)
         (tint: XnaColor) =
         let draw position sprite =
             drawWorldTile spriteBatch textures camera viewportWidth viewportHeight position sprite tint
+        let rippleFrame = waterFrameTick tick
 
         let tx0 = max 0 (int camera.X / Sim.tilePixels)
         let ty0 = max 0 (int camera.Y / Sim.tilePixels)
@@ -366,7 +382,7 @@ module VillageArt =
             let position = worldTile plan localPosition
             let sprite =
                 match kind with
-                | Water -> WaterSprite
+                | Water -> if rippleFrame = 0 then WaterSprite else WaterSpriteB
                 | StonePath -> StonePathSprite
             draw position sprite
 
@@ -383,7 +399,8 @@ module VillageArt =
         let drawBridge localPosition =
             let position = worldTile plan localPosition
             draw position BridgeSprite
-            draw (offsetTile position 1 0) BridgeSprite
+            draw (offsetTile position 0 1) BridgeSprite
+            draw (offsetTile position 0 2) BridgeSprite
 
         let drawBamboo localPosition =
             let position = worldTile plan localPosition