feat(mapgen): 参数化确定性生成器与视口裁剪（P20 第一步）

- 新增纯函数 MapGen：尺寸参数化（默认 64x48，可 256x192），splitmix64
  随机游走河道 + 2-3 octave value noise + 泊松式布点；核心区整平后以 BFS
  校验 5x6=30 个 spawn 全部可达，不可达确定性修整并复验。
- visibleTileRange/visibleTileCount 纯函数，drawWorld 改为调用之（行为不变）；
  drawMapFitted/drawMapViewport 仅证据用。
- 存档沿用 v2/v3 边界字段，补参数化边界 round-trip 测试，未改 Sim 语义。
- 回归：Desktop 109->115、Kernel 89->90；final_digest 与基线一致。

1 files changed, 63 insertions, 4 deletions

diff --git a/src/LivingVillage.Desktop/VillageArt.fs b/src/LivingVillage.Desktop/VillageArt.fs
index 11a6d71..51e17e0 100644
--- a/src/LivingVillage.Desktop/VillageArt.fs
+++ b/src/LivingVillage.Desktop/VillageArt.fs
@@ -643,10 +643,8 @@ module VillageArt =
             | Some cc0 -> drawCc0Tile spriteBatch cc0 tint (cc0GroundRect position) (position.X * Sim.tilePixels - int camera.X) (position.Y * Sim.tilePixels - int camera.Y)
             | None -> draw position GrassSprite
 
-        let tx0 = max 0 (int camera.X / Sim.tilePixels)
-        let ty0 = max 0 (int camera.Y / Sim.tilePixels)
-        let tx1 = min (Sim.mapWidthTiles - 1) ((int camera.X + viewportWidth) / Sim.tilePixels + 1)
-        let ty1 = min (Sim.mapHeightTiles - 1) ((int camera.Y + viewportHeight) / Sim.tilePixels + 1)
+        let tx0, ty0, tx1, ty1 =
+            MapGen.visibleTileRange Sim.mapWidthTiles Sim.mapHeightTiles (int camera.X) (int camera.Y) viewportWidth viewportHeight
         for y in ty0 .. ty1 do
             for x in tx0 .. tx1 do
                 drawBaseTile (tilePosition x y)
@@ -724,6 +722,67 @@ module VillageArt =
             | House -> draw (offsetTile (worldTile plan prop.Position) 0 -3) smokeSprite
             | _ -> ()
 
+    /// 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 drawMapFitted
+        (spriteBatch: SpriteBatch)
+        (textures: ArtTextures)
+        (viewportWidth: int)
+        (viewportHeight: int)
+        (map: MapGen.Result)
+        (tint: XnaColor) : int =
+        let scale =
+            min (float32 viewportWidth / float32 (max 1 (map.Width * Sim.tilePixels)))
+                (float32 viewportHeight / float32 (max 1 (map.Height * Sim.tilePixels)))
+        let cell = max 1 (int (float32 Sim.tilePixels * scale))
+        let offsetX = (viewportWidth - map.Width * cell) / 2
+        let offsetY = (viewportHeight - map.Height * cell) / 2
+        let spriteOf code =
+            match code with
+            | 1 -> WaterSprite
+            | 2
+            | 3 -> StonePathSprite
+            | _ -> GrassSprite
+        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)
+        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)
+        (camera: XnaVector2)
+        (viewportWidth: int)
+        (viewportHeight: int)
+        (map: MapGen.Result)
+        (tick: int64)
+        (tint: XnaColor) : int =
+        let x0, y0, x1, y1 =
+            MapGen.visibleTileRange map.Width map.Height (int camera.X) (int camera.Y) viewportWidth viewportHeight
+        let spriteOf (x: int) (y: int) code =
+            match code with
+            | 1 ->
+                match waterFrameTickAt tick x y with
+                | 0 -> WaterSprite
+                | 1 -> WaterSpriteB
+                | _ -> WaterSpriteC
+            | 2
+            | 3 -> StonePathSprite
+            | _ -> GrassSprite
+        let mutable drawn = 0
+        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)
+                drawn <- drawn + 1
+        drawn
+
     let drawCharacter
         (spriteBatch: SpriteBatch)
         (textures: ArtTextures)