feat(mapgen): 河道连续性不变量 + 桥/石板路网/沿河民居（P20 第二步）p20-map-gen-foundation

- 河道改为贯穿全图连续水带（每列一段、宽度 >= RiverWidth），置于核心区上下
  两侧且不相交；新增不变量测试：逐列水段数恒定、每段宽度下限、任意水格
  >=2 个水邻居（禁止断裂缺口与孤立水点）。
- Result 新增 Rivers/Bridges/Paths/Buildings/BridgeCrossingsOk；每河 2 座桥、
  横贯主路 + 垂直连接路 + 远岸路 + 白墙黑瓦民居（门朝路）。
- 公开 isWalkable/floodFill；ReachabilityOk 现覆盖 30 spawn + 所有门 + 所有桥面，
  BridgeCrossingsOk 保证每桥两岸同连通域。
- 回归：Desktop 115->119、Kernel 90 不变；final_digest 与基线一致。

1 files changed, 34 insertions, 6 deletions

diff --git a/src/LivingVillage.Desktop/VillageArt.fs b/src/LivingVillage.Desktop/VillageArt.fs
index 51e17e0..19c9aa6 100644
--- a/src/LivingVillage.Desktop/VillageArt.fs
+++ b/src/LivingVillage.Desktop/VillageArt.fs
@@ -724,9 +724,34 @@ module VillageArt =
 
     /// P20 证据用：把整张 MapGen 地图等比缩放到视口内绘制，返回绘制的瓦片数。
     /// 仅用于 `LV_MAPGEN_SHOT` 整图取证，不参与正常游戏循环。
+    let private fillRect (spriteBatch: SpriteBatch) (pixel: Texture2D) (color: XnaColor) (x: int) (y: int) (w: int) (h: int) =
+        spriteBatch.Draw(pixel, XnaRectangle(x, y, max 1 w, max 1 h), color)
+
+    /// 整图证据里叠加桥面与沿河民居（白墙黑瓦色块 + 门）。
+    let private drawMapStructures
+        (spriteBatch: SpriteBatch)
+        (textures: ArtTextures)
+        (pixel: Texture2D)
+        (tileX: int -> int)
+        (tileY: int -> int)
+        (cell: int)
+        (map: MapGen.Result) =
+        let bridgeSource = tileSourceRectangle BridgeSprite
+        for (x, y) in map.Bridges do
+            spriteBatch.Draw(textures.TileAtlas, XnaRectangle(tileX x, tileY y, cell, cell), bridgeSource, XnaColor.White)
+        for building in map.Buildings do
+            for row in building.Top .. building.Top + building.Height - 1 do
+                // 白墙朝向道路一行（门所在行），其余为黑瓦屋顶，两侧河岸朝向一致。
+                let color =
+                    if row = building.DoorY then XnaColor(236, 232, 220)
+                    else XnaColor(48, 52, 60)
+                fillRect spriteBatch pixel color (tileX building.Left) (tileY row) (building.Width * cell) (cell)
+            fillRect spriteBatch pixel (XnaColor(96, 60, 42)) (tileX building.DoorX) (tileY building.DoorY) cell cell
+
     let drawMapFitted
         (spriteBatch: SpriteBatch)
         (textures: ArtTextures)
+        (pixel: Texture2D)
         (viewportWidth: int)
         (viewportHeight: int)
         (map: MapGen.Result)
@@ -737,6 +762,8 @@ module VillageArt =
         let cell = max 1 (int (float32 Sim.tilePixels * scale))
         let offsetX = (viewportWidth - map.Width * cell) / 2
         let offsetY = (viewportHeight - map.Height * cell) / 2
+        let tileX x = offsetX + x * cell
+        let tileY y = offsetY + y * cell
         let spriteOf code =
             match code with
             | 1 -> WaterSprite
@@ -746,13 +773,15 @@ module VillageArt =
         for y in 0 .. map.Height - 1 do
             for x in 0 .. map.Width - 1 do
                 let code = map.Tiles.[y * map.Width + x]
-                spriteBatch.Draw(textures.TileAtlas, XnaRectangle(offsetX + x * cell, offsetY + y * cell, cell, cell), tileSourceRectangle (spriteOf code), tint)
+                spriteBatch.Draw(textures.TileAtlas, XnaRectangle(tileX x, tileY y, cell, cell), tileSourceRectangle (spriteOf code), tint)
+        drawMapStructures spriteBatch textures pixel tileX tileY cell map
         map.Width * map.Height
 
     /// P20 证据用：按相机做视口裁剪，只绘制可见瓦片（1:1 像素），返回实际绘制数。
     let drawMapViewport
         (spriteBatch: SpriteBatch)
         (textures: ArtTextures)
+        (pixel: Texture2D)
         (camera: XnaVector2)
         (viewportWidth: int)
         (viewportHeight: int)
@@ -761,6 +790,8 @@ module VillageArt =
         (tint: XnaColor) : int =
         let x0, y0, x1, y1 =
             MapGen.visibleTileRange map.Width map.Height (int camera.X) (int camera.Y) viewportWidth viewportHeight
+        let tileX x = x * Sim.tilePixels - int camera.X
+        let tileY y = y * Sim.tilePixels - int camera.Y
         let spriteOf (x: int) (y: int) code =
             match code with
             | 1 ->
@@ -775,12 +806,9 @@ module VillageArt =
         for y in y0 .. y1 do
             for x in x0 .. x1 do
                 let code = map.Tiles.[y * map.Width + x]
-                spriteBatch.Draw(
-                    textures.TileAtlas,
-                    XnaRectangle(x * Sim.tilePixels - int camera.X, y * Sim.tilePixels - int camera.Y, Sim.tilePixels, Sim.tilePixels),
-                    tileSourceRectangle (spriteOf x y code),
-                    tint)
+                spriteBatch.Draw(textures.TileAtlas, XnaRectangle(tileX x, tileY y, Sim.tilePixels, Sim.tilePixels), tileSourceRectangle (spriteOf x y code), tint)
                 drawn <- drawn + 1
+        drawMapStructures spriteBatch textures pixel tileX tileY Sim.tilePixels map
         drawn
 
     let drawCharacter