fix(render): doorway blue-hole — render root clobbered by NPCs (CurrCell per-entity write)

THE doorway flap root cause, found via [flap-cam]/[shell]/[cell-transit] (2026-06-03):
the player spawned + stood still in the room (cell 0171, NO [cell-transit] after teleport),
yet the render rooted at the vestibule (0170) for all 77,951 frames — drawing only 0170's
~8-triangle shell, the rest = GL clear color = the bluish void.

CellGraph.CurrCell IS "the player's cell" (the render root), but it was written by
SetCurrAndReturn inside the PER-ENTITY ResolveWithTransition + ResolveCellId — so EVERY NPC
wrote it. A Holtburg NPC (0x000F4240) jump-looping near the doorway clobbered the player's
render root every tick. Standing still (player makes no resolve calls) the NPC's write wins
→ stuck blue void; moving, player/NPC writes fight → the flap. This is why the membership
pick fix (correct, kept) didn't change the visual — the render root was clobbered regardless.

Fix: CurrCell is now written ONLY by the player. New PhysicsEngine.UpdatePlayerCurrCell is
called from PlayerMovementController.UpdateCellId — the single player-only chokepoint for
CellId (teleport / server snap @ SetPosition + per-frame resolver). Removed the CurrCell
write from SetCurrAndReturn (inlined the 2 resolve call sites to sp.CurCellId) and the 4
ResolveCellId sites. NPCs no longer touch the render root. Teleport→UpdateCellId also covers
spawn/standing-still (CurrCell = the player's spawn cell immediately).

CellGraphMembershipTests rewritten to the new contract (3 tests): UpdatePlayerCurrCell writes
the render root; ResolveCellId does NOT (the blue-hole guard); stale-beats-null preserved.
Full Core suite: 1295 pass / 5 fail = the documented §10 baseline, zero new breakage.

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

tests/AcDream.Core.Tests/Physics/CellGraphMembershipTests.cs

@@ -1,39 +1,77 @@
 using System.Collections.Generic;
 using System.Numerics;
 using AcDream.Core.Physics;
-using AcDream.Core.World.Cells;
 using DatReaderWriter.Types;
 using Xunit;
 using DatEnvCell = DatReaderWriter.DBObjs.EnvCell;
 
 namespace AcDream.Core.Tests.Physics;
 
+// 2026-06-03 blue-hole fix: CellGraph.CurrCell IS "the player's cell" (CellGraph.cs:19) — it roots
+// the indoor render (GameWindow.OnRender). It is written ONLY by the player-only
+// PhysicsEngine.UpdatePlayerCurrCell (called from PlayerMovementController.UpdateCellId, the single
+// player chokepoint for CellId), NOT by the per-entity ResolveCellId / ResolveWithTransition. Before
+// the fix, those per-entity paths wrote CurrCell, so a Holtburg NPC jump-looping near the cottage
+// doorway clobbered the player's render root every tick → the render rooted at the NPC's tiny
+// connector cell → the cottage doorway "blue-hole" flap.
 public class CellGraphMembershipTests
 {
-    [Fact]
-    public void ResolveCellId_Resolved_WritesCurrCellTrackingTheResolvedId()
+    private static PhysicsEngine MakeEngineWithCell(uint cellId)
     {
         var engine = new PhysicsEngine();
         var cache = new PhysicsDataCache();
         engine.DataCache = cache;
 
-        var cs = new CellStruct {
+        var cs = new CellStruct
+        {
             VertexArray = new VertexArray { Vertices = new Dictionary<ushort, SWVertex>() },
             Polygons = new Dictionary<ushort, Polygon>(),
             PhysicsBSP = null,
         };
-        var dat = new DatEnvCell {
+        var dat = new DatEnvCell
+        {
             Flags = (DatReaderWriter.Enums.EnvCellFlags)0,
             CellPortals = new List<DatReaderWriter.Types.CellPortal>(),
             VisibleCells = new List<ushort>(),
         };
-        cache.CacheCellStruct(0xA9B40174u, dat, cs, Matrix4x4.Identity);  // registers in the graph (W1)
+        cache.CacheCellStruct(cellId, dat, cs, Matrix4x4.Identity);  // registers in the graph (W1)
+        return engine;
+    }
+
+    [Fact]
+    public void UpdatePlayerCurrCell_Resolved_WritesCurrCellTrackingTheId()
+    {
+        var engine = MakeEngineWithCell(0xA9B40174u);
+
+        engine.UpdatePlayerCurrCell(0xA9B40174u);
+
+        Assert.NotNull(engine.DataCache!.CellGraph.CurrCell);
+        Assert.Equal(0xA9B40174u, engine.DataCache.CellGraph.CurrCell!.Id);
+    }
+
+    [Fact]
+    public void UpdatePlayerCurrCell_UnresolvableId_LeavesCurrCellUnchanged()
+    {
+        var engine = MakeEngineWithCell(0xA9B40174u);
+        engine.UpdatePlayerCurrCell(0xA9B40174u);   // CurrCell = 0174
+
+        engine.UpdatePlayerCurrCell(0xDEADBEEFu);   // not in the graph → stale beats null
+
+        Assert.NotNull(engine.DataCache!.CellGraph.CurrCell);
+        Assert.Equal(0xA9B40174u, engine.DataCache.CellGraph.CurrCell!.Id);
+    }
+
+    // Guards the blue-hole fix: the per-entity ResolveCellId must NOT touch the render root.
+    // (Only the player's UpdatePlayerCurrCell does.) Before the fix this wrote CurrCell, so a
+    // nearby NPC's ResolveCellId/ResolveWithTransition clobbered the player's render root.
+    [Fact]
+    public void ResolveCellId_DoesNotWriteTheRenderRoot()
+    {
+        var engine = MakeEngineWithCell(0xA9B40174u);
 
         uint result = engine.ResolveCellId(new Vector3(0, 0, 0), 0.5f, 0xA9B40174u);
 
-        // CurrCell tracks whatever id ResolveCellId returned (when that id is in the graph).
-        Assert.NotNull(cache.CellGraph.CurrCell);
-        Assert.Equal(result, cache.CellGraph.CurrCell!.Id);
-        Assert.Equal(0xA9B40174u, cache.CellGraph.CurrCell!.Id);
+        Assert.Equal(0xA9B40174u, result);                    // still resolves the cell correctly
+        Assert.Null(engine.DataCache!.CellGraph.CurrCell);    // but does NOT write the render root
     }
 }