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, 32 insertions, 0 deletions

diff --git a/src/LivingVillage.Desktop.Tests/MapGenTests.fs b/src/LivingVillage.Desktop.Tests/MapGenTests.fs
index 4d11dd1..4a09865 100644
--- a/src/LivingVillage.Desktop.Tests/MapGenTests.fs
+++ b/src/LivingVillage.Desktop.Tests/MapGenTests.fs
@@ -229,6 +229,38 @@ type MapGenTests () =
             Assert.IsTrue(visited.[y * map.Width + x], sprintf "bridge tile (%d,%d) unreachable at 512x384" x y)
         Assert.IsTrue(map.Buildings.Length >= map.Rivers.Length, "riverside building per band at 512x384")
 
+    // ---- P24：512x384 沿河民居加密（默认图不受影响） ----
+
+    [<TestMethod>]
+    member _.ExtraHouseDensityOnlyEnabledForLargeMaps () =
+        Assert.AreEqual<int>(0, (MapGen.paramsForSize 64 48 (uint64 1)).ExtraRiversideHouses)
+        Assert.AreEqual<int>(0, (MapGen.paramsForSize 256 192 (uint64 1)).ExtraRiversideHouses)
+        Assert.IsTrue((MapGen.paramsForSize 512 384 (uint64 1)).ExtraRiversideHouses > 0)
+
+    [<TestMethod>]
+    member _.LargeMapHasAtLeastTwelveHousesWithReachableDoors () =
+        let map = MapGen.generateWithSize 512 384 (uint64 4242)
+        Assert.IsTrue(map.Buildings.Length >= 12, sprintf "512x384 must place >=12 houses, saw %d" map.Buildings.Length)
+        let startX, startY = map.Spawns.Head
+        let visited = MapGen.floodFill map startX startY
+        for building in map.Buildings do
+            Assert.IsTrue(MapGen.isWalkable map building.DoorX building.DoorY, sprintf "door (%d,%d) must be walkable" building.DoorX building.DoorY)
+            Assert.IsTrue(visited.[building.DoorY * map.Width + building.DoorX], sprintf "door (%d,%d) unreachable" building.DoorX building.DoorY)
+        Assert.IsTrue(map.ReachabilityOk, "512x384 reachability must hold with extra houses")
+
+    [<TestMethod>]
+    member _.LargeMapHousePlacementIsDeterministic () =
+        let a = MapGen.generateWithSize 512 384 (uint64 4242)
+        let b = MapGen.generateWithSize 512 384 (uint64 4242)
+        Assert.AreEqual<int>(a.Buildings.Length, b.Buildings.Length)
+        Assert.IsTrue(a.Buildings = b.Buildings)
+        Assert.AreEqual<string>(MapGen.serialize a, MapGen.serialize b)
+
+    [<TestMethod>]
+    member _.DefaultSizeHouseCountUnchanged () =
+        Assert.AreEqual<int>(4, (MapGen.generateWithSize 64 48 (uint64 4242)).Buildings.Length)
+        Assert.AreEqual<int>(4, (MapGen.generateWithSize 256 192 (uint64 4242)).Buildings.Length)
+
     /// 锁定当前默认尺寸的生成结果，防止 P23 参数适配无意改变 64x48 / 256x192。
     [<TestMethod>]
     member _.DefaultSizeChecksumsArePinned () =