feat(mapgen): 512x384 沿河民居加密到 18 座且门全部可达（P24）

- Params.ExtraRiversideHouses（默认 0）；paramsForSize 仅对 >256x192 设为 10，
  从既有路网泊松散布 3x2 民居并接门入网（structureRng 确定性），512x384 共 18 座。
- 64x48/256x192 仍 4 座、checksum 6042052/1284656962 不变（新增用例锁定）。
- 新增 512 民居数量/可达/确定性用例；Desktop 130 -> 134。

1 files changed, 52 insertions, 3 deletions

diff --git a/src/LivingVillage.Desktop/MapGen.fs b/src/LivingVillage.Desktop/MapGen.fs
index ce9bccf..1e63ed2 100644
--- a/src/LivingVillage.Desktop/MapGen.fs
+++ b/src/LivingVillage.Desktop/MapGen.fs
@@ -30,7 +30,9 @@ module MapGen =
           RiverWidth: int
           CoreSide: int
           SpawnColumns: int
-          SpawnRows: int }
+          SpawnRows: int
+          /// P24：大图额外沿河民居数量（沿现有路网泊松散布）。默认 0，64x48/256x192 不受影响。
+          ExtraRiversideHouses: int }
 
     /// P20 第二步：一条贯穿全图的东西向河道（每列恰好一段，宽度下限 RiverWidth）。
     type River =
@@ -70,16 +72,20 @@ module MapGen =
           RiverWidth = 3
           CoreSide = 14
           SpawnColumns = 5
-          SpawnRows = 6 }
+          SpawnRows = 6
+          ExtraRiversideHouses = 0 }
 
     /// P23 尺寸自适应参数：河道数量随高度增长（约每 96 行一条），其余沿用默认。
     /// 64x48 / 256x192 仍得到 2 条河道，与既有输出逐字节一致；512x384 得到 4 条。
+    /// P24：仅对大于 256x192 的图追加沿河民居（泊松散布），保证既有 checksum 不变。
     let paramsForSize (width: int) (height: int) (seed: uint64) : Params =
         let base_ = defaultParams seed
+        let large = width > 256 || height > 192
         { base_ with
             Width = width
             Height = height
-            RiverCount = max base_.RiverCount (max 1 (height / 96)) }
+            RiverCount = max base_.RiverCount (max 1 (height / 96))
+            ExtraRiversideHouses = (if large then 10 else 0) }
 
     // ---- splitmix64 与 value noise（无外部依赖、无时钟） ----
 
@@ -275,6 +281,49 @@ module MapGen =
             buildings <- building :: buildings
             addPath building.DoorX building.DoorY
 
+        // 5b) P24：大图沿河民居加密。从既有路网瓦片出发，向四邻定向落位 3x2 白墙黛瓦民居，
+        //     门开向相邻路格并接入路网（BFS 可达）；仅在 ExtraRiversideHouses>0 时执行，
+        //     且只在核心区外的草地落位，故 64x48/256x192 输出逐字节不变。
+        if p.ExtraRiversideHouses > 0 then
+            let footprintFree (left: int) (top: int) =
+                left >= 1 && left + 2 <= w - 2 && top >= 1 && top + 1 <= h - 2
+                && [ for dx in 0 .. 2 do
+                       for dy in 0 .. 1 -> (left + dx, top + dy) ]
+                   |> List.forall (fun (x, y) ->
+                       tiles.[idx x y] = int GroundTile.Grass
+                       && not (inCore x y))
+            let overlaps (left: int) (top: int) =
+                buildings
+                |> List.exists (fun b ->
+                    left < b.Left + b.Width && b.Left < left + 3
+                    && top < b.Top + b.Height && b.Top < top + 2)
+            let directions = [ (0, -1); (0, 1); (-1, 0); (1, 0) ]
+            let mutable extraPlaced = 0
+            let mutable extraAttempts = 0
+            let maxAttempts = p.ExtraRiversideHouses * 400
+            while extraPlaced < p.ExtraRiversideHouses && extraAttempts < maxAttempts && paths.Length > 0 do
+                extraAttempts <- extraAttempts + 1
+                let nextState, raw = splitmix structureRng
+                structureRng <- nextState
+                if paths.Length > 0 then
+                    let pathX, pathY = List.item (int (raw % uint64 paths.Length)) paths
+                    let dirX, dirY = directions.[int ((raw >>> 32) % 4UL)]
+                    let doorX = pathX + dirX
+                    let doorY = pathY + dirY
+                    let left = doorX - 1
+                    let top = doorY - 1
+                    if footprintFree left top && not (overlaps left top) then
+                        let building =
+                            { Left = left
+                              Top = top
+                              Width = 3
+                              Height = 2
+                              DoorX = doorX
+                              DoorY = doorY }
+                        buildings <- building :: buildings
+                        addPath building.DoorX building.DoorY
+                        extraPlaced <- extraPlaced + 1
+
         // 6) 30 个 spawn：核心区内的规则网格。
         let sc = max 2 p.SpawnColumns
         let sr = max 2 p.SpawnRows