feat(mapgen): 512x384 尺寸自适应参数与扩程回归（P23）

- paramsForSize：河道数量随高度约每 96 行一条（64x48/256x192 仍 2 条，512x384 得 4 条），
  第 3 条起在核心上/下方逐层外扩，互不重叠且不压核心。
- 噪声特征尺度封顶 4x，避免 512x384 退化成过度平滑的放大图。
- 64x48/256x192 生成结果逐字节不变（新增 checksum 锁定用例）。
- 新增 512x384 同 seed 逐字节一致/异 seed 分化/四河道连续/30 spawn 与桥两岸可达用例。
- Desktop 测试 124 -> 130。

1 files changed, 30 insertions, 5 deletions

diff --git a/src/LivingVillage.Desktop/MapGen.fs b/src/LivingVillage.Desktop/MapGen.fs
index 7c7059a..ce9bccf 100644
--- a/src/LivingVillage.Desktop/MapGen.fs
+++ b/src/LivingVillage.Desktop/MapGen.fs
@@ -72,6 +72,15 @@ module MapGen =
           SpawnColumns = 5
           SpawnRows = 6 }
 
+    /// 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。
+    let paramsForSize (width: int) (height: int) (seed: uint64) : Params =
+        let base_ = defaultParams seed
+        { base_ with
+            Width = width
+            Height = height
+            RiverCount = max base_.RiverCount (max 1 (height / 96)) }
+
     // ---- splitmix64 与 value noise（无外部依赖、无时钟） ----
 
     let private splitmix (state: uint64) : uint64 * uint64 =
@@ -104,6 +113,10 @@ module MapGen =
         (v00 * (1.0f - sx) + v10 * sx) * (1.0f - sy) + (v01 * (1.0f - sx) + v11 * sx) * sy
 
     /// 2-3 octave 叠加；特征尺度随地图宽度等比缩放，保证小图/大图观感一致。
+    /// P23：特征尺度封顶（<=4x），避免 512x384 退化成过于平滑的「放大图」。
+    let private noiseWidthScale (width: int) : float32 =
+        min 4.0f (float32 width / 64.0f)
+
     let private multiOctave (widthScale: float32) (x: int) (y: int) (seed: uint64) : float32 =
         let xf = float32 x
         let yf = float32 y
@@ -138,7 +151,7 @@ module MapGen =
             x >= core.MinX && x <= core.MaxX && y >= core.MinY && y <= core.MaxY
 
         // 1) 多八度噪声：低洼草地转泥炭（跳过核心区）。
-        let widthScale = float32 w / 64.0f
+        let widthScale = noiseWidthScale w
         for y in 0 .. h - 1 do
             for x in 0 .. w - 1 do
                 if multiOctave widthScale x y p.Seed < 0.18f && not (inCore x y) then
@@ -148,14 +161,26 @@ module MapGen =
         let riverWidth = max 3 p.RiverWidth
         let riverBands =
             [ for i in 0 .. max 0 p.RiverCount - 1 do
-                if i % 2 = 0 then
+                if i = 0 then
                     let centerY = max 1 (min (h - 1 - riverWidth) (min (core.MinY - riverWidth - 2) (h / 4)))
                     if centerY + riverWidth - 1 < core.MinY then
                         yield { CenterY = centerY; Width = riverWidth }
-                else
+                elif i = 1 then
                     let centerY = max 1 (min (h - 1 - riverWidth) (max (core.MaxY + 2) (3 * h / 4)))
                     if centerY > core.MaxY then
-                        yield { CenterY = centerY; Width = riverWidth } ]
+                        yield { CenterY = centerY; Width = riverWidth }
+                else
+                    // P23：第 3 条起在核心上/下方逐层外扩堆叠，互不重叠、不压核心。
+                    let layer = (i - 1) / 2
+                    let offset = if i % 2 = 1 then layer - 1 else layer
+                    if i % 2 = 0 then
+                        let centerY = core.MinY - riverWidth - 2 - offset * (riverWidth + 3)
+                        if centerY >= 1 then
+                            yield { CenterY = centerY; Width = riverWidth }
+                    else
+                        let centerY = core.MaxY + 2 + offset * (riverWidth + 3)
+                        if centerY + riverWidth - 1 <= h - 2 then
+                            yield { CenterY = centerY; Width = riverWidth } ]
         for river in riverBands do
             for x in 1 .. w - 2 do
                 for dy in 0 .. river.Width - 1 do
@@ -310,7 +335,7 @@ module MapGen =
           BridgeCrossingsOk = crossingsOk }
 
     let generateWithSize (width: int) (height: int) (seed: uint64) : Result =
-        generate { defaultParams seed with Width = width; Height = height }
+        generate (paramsForSize width height seed)
 
     /// 生成结果的确定性序列化（同 seed 逐字节一致），供回归测试与跨机核对。
     let serialize (map: Result) : string =