fix(physics): activate collision generations atomically

src/AcDream.Core/World/Cells/CellGraph.cs

@@ -125,4 +125,76 @@ public sealed class CellGraph
                 return stab;
         return null;
     }
+
+    /// <summary>
+    /// Creates an immutable-reference snapshot for collision-generation
+    /// preparation. EnvCell and TerrainSurface records are immutable after
+    /// publication, so copying the registries is sufficient; the active graph
+    /// remains untouched while the staging graph is rebuilt.
+    /// </summary>
+    internal CellGraph CreateCollisionStagingCopy()
+    {
+        var copy = new CellGraph { CurrCell = CurrCell };
+        foreach ((uint id, EnvCell cell) in _envCells)
+            copy._envCells.TryAdd(id, cell);
+        foreach ((uint id, (TerrainSurface Terrain, Vector3 Origin) terrain) in
+                 _terrain)
+        {
+            copy._terrain.TryAdd(id, terrain);
+        }
+        return copy;
+    }
+
+    internal PreparedCellGraphLandblock PrepareLandblockReplacement(
+        uint landblockId)
+    {
+        uint prefix = landblockId & 0xFFFF0000u;
+        KeyValuePair<uint, EnvCell>[] envCells = _envCells
+            .Where(static pair => (pair.Key & 0xFFFFu) >= 0x0100u)
+            .Where(pair => (pair.Key & 0xFFFF0000u) == prefix)
+            .OrderBy(static pair => pair.Key)
+            .ToArray();
+        bool hasTerrain = _terrain.TryGetValue(prefix, out var terrain);
+        return new PreparedCellGraphLandblock(
+            prefix,
+            envCells,
+            hasTerrain,
+            terrain.Terrain,
+            terrain.Origin,
+            CurrCell?.Id ?? 0u);
+    }
+
+    internal void CommitLandblockReplacement(
+        PreparedCellGraphLandblock replacement)
+    {
+        uint currentCellId = CurrCell?.Id ?? 0u;
+        RemoveLandblock(replacement.LandblockPrefix);
+        if (replacement.HasTerrain)
+        {
+            _terrain[replacement.LandblockPrefix] = (
+                replacement.Terrain!,
+                replacement.Origin);
+        }
+        foreach ((uint id, EnvCell cell) in replacement.EnvCells)
+            _envCells[id] = cell;
+
+        uint desiredCurrentCellId =
+            (currentCellId & 0xFFFF0000u) == replacement.LandblockPrefix
+                ? currentCellId
+                : currentCellId == 0u
+                    && (replacement.CurrentCellId & 0xFFFF0000u)
+                        == replacement.LandblockPrefix
+                        ? replacement.CurrentCellId
+                        : 0u;
+        if (desiredCurrentCellId != 0u)
+            CurrCell = GetVisible(desiredCurrentCellId);
+    }
 }
+
+internal sealed record PreparedCellGraphLandblock(
+    uint LandblockPrefix,
+    KeyValuePair<uint, EnvCell>[] EnvCells,
+    bool HasTerrain,
+    TerrainSurface? Terrain,
+    Vector3 Origin,
+    uint CurrentCellId);