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, 22 insertions, 0 deletions

diff --git a/src/LivingVillage.Kernel.Tests/M6aTests.fs b/src/LivingVillage.Kernel.Tests/M6aTests.fs
index fc8cc3a..9348b0f 100644
--- a/src/LivingVillage.Kernel.Tests/M6aTests.fs
+++ b/src/LivingVillage.Kernel.Tests/M6aTests.fs
@@ -292,3 +292,25 @@ type M6aTests () =
         Assert.AreEqual<string>("渔夫", Occupation.nameOf Occupation.Fisher)
         Assert.AreEqual<string>("货郎", Occupation.nameOf Occupation.Peddler)
         Assert.AreEqual<string>("书生", Occupation.nameOf Occupation.Scholar)
+
+    /// P20：存档 v2 的边界字段在非默认尺寸下必须 round-trip，读回后
+    /// mapWidthTiles/mapHeightTiles 恢复且再次保存与原文逐字节一致。
+    /// 旧 64x48 存档的 Round-trip 保持不变由既有 v1/v2 用例覆盖。
+    [<TestMethod>]
+    member _.SaveRoundTripsParameterizedMapBounds () =
+        let path = "p20-bounds-v2.save"
+        try
+            Sim.configureBounds 256 192
+            let world = Sim.initialWorld (uint64 4242)
+            WorldSave.saveToFile path world
+            let text = File.ReadAllText path
+            Assert.IsTrue(text.StartsWith("LV_WORLD_SAVE_V2"), "bounds save must stay v2")
+            match WorldSave.loadFromFile path with
+            | Error failure -> Assert.Fail(sprintf "bounds save should load: %s" failure)
+            | Ok restored ->
+                Assert.AreEqual<int>(256, Sim.mapWidthTiles)
+                Assert.AreEqual<int>(192, Sim.mapHeightTiles)
+                Assert.AreEqual<string>(text, WorldSave.save restored)
+        finally
+            Sim.configureBounds 64 48
+            if File.Exists path then File.Delete path