feat(core): UCG Stage 1 — populate CellGraph from CacheCellStruct + AddLandblock (inert)

PhysicsDataCache gains a `CellGraph` property (UCG Stage 1). The env-cell
hook is placed at the very top of CacheCellStruct — before the idempotency
guard and the null-PhysicsBSP early-return — so BSP-less cells are included
in the graph even though they are dropped from the legacy _cellStruct map.
PhysicsEngine.AddLandblock/RemoveLandblock mirror terrain registration into
the graph via a null-guarded DataCache?.CellGraph call. Zero behavior change:
CellGraph has no readers this stage.

A using-alias (UcgEnvCell / UcgCellGraph) resolves the EnvCell name
collision between AcDream.Core.World.Cells and DatReaderWriter.DBObjs.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -60,6 +60,9 @@ public sealed class PhysicsEngine
         float worldOffsetX, float worldOffsetY)
     {
         _landblocks[landblockId] = new LandblockPhysics(terrain, cells, portals, worldOffsetX, worldOffsetY);
+
+        // UCG Stage 1: mirror terrain into the unified graph (inert this stage).
+        DataCache?.CellGraph.RegisterTerrain(landblockId, terrain, new Vector3(worldOffsetX, worldOffsetY, 0f));
     }
 
     /// <summary>
@@ -69,6 +72,9 @@ public sealed class PhysicsEngine
     {
         _landblocks.Remove(landblockId);
         ShadowObjects.RemoveLandblock(landblockId);
+
+        // UCG Stage 1: mirror removal into the unified graph (inert this stage).
+        DataCache?.CellGraph.RemoveLandblock(landblockId);
     }
 
     /// <summary>