feat(render): indoor render WORKS — terminating portal flood + every-cell seal + look-in FPS

Checkpoint of the unified retail-faithful indoor render. The two-week HANG/grey is fixed and the
interior seals (live-verified by the user). Commits the session render-rewrite foundation together
with the fixes that made it functional.

- HANG fix: PortalVisibilityBuilder.Build portal flood did not terminate (the faithful ProjectToClip
  near-side clip drifts per round, defeating the CellView dedup; the BFS had no bound after U.2a removed
  MaxReprocessPerCell). Fix = drift-tolerant snapped/canonical CellView.Add dedup (PortalView.cs) plus
  restored MaxReprocessPerCell=16 bounded re-enqueue (PortalVisibilityBuilder.cs). Re-enqueue is kept
  (load-bearing for late-slice propagation, Build_ViewGrowthAfterDoneCell_PropagatesNewSlicesToExit);
  only its count is capped. CellViewDedupTests added.
- Seal (DrawCells Task 2): RetailPViewRenderer.DrawEnvCellShells draws EVERY visible cell via
  IndoorDrawPlan.ShellPass (was gated on the ClipFrameAssembler slot filter, leaving slot-less cells grey).
- Look-in FPS: GameWindow exterior look-in candidates limited to the player landblock +-1 (was all ~81
  loaded LBs iterated every outdoor frame). No behaviour change (far cells were >48m, already culled).

Remaining dominant issue = the FLAP at transitions: viewer-cell metastability (render roots at the
camera-eye cell, which oscillates outdoor-indoor as the 3rd-person boom drifts across the doorway,
confirmed in render-sig). SEPARATE fix, NOT the DrawCells port. Full handoff + flap fix plan + tracked
follow-ups (#78 terrain, look-in-from-inside, look-in FPS, L-spotlight):
docs/research/2026-06-07-indoor-render-session-handoff.md.

Baselines: build 0 err; App.Tests 210/210; Core.Tests 1331 pass / 4 fail (pre-existing) / 1 skip.

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

src/AcDream.Core/World/WorldEntity.cs

@@ -37,12 +37,13 @@ public sealed class WorldEntity
     public PaletteOverride? PaletteOverride { get; init; }
 
     /// <summary>
-    /// EnvCell ID that owns this entity (room geometry or static object inside
+    /// EnvCell or outdoor cell ID that owns this entity (room geometry, static
+    /// object, or live object inside/outside a cell).
     /// the cell). Used by portal visibility to filter interior entities — only
     /// entities whose ParentCellId appears in the visible set are rendered.
-    /// Null for outdoor entities (stabs, scenery, live server spawns).
+    /// Null for outdoor dat scenery/building stabs or unresolved live entities.
     /// </summary>
-    public uint? ParentCellId { get; init; }
+    public uint? ParentCellId { get; set; }
 
     /// <summary>
     /// True when this entity originates from <c>LandBlockInfo.Buildings[]</c>