fix(render): Phase A8 RR7.1 — stamp BuildingId on cells loaded across multiple frames

RR7 visual gate (2026-05-27) revealed the indoor branch NEVER fired even
when the strict gate's PointInCell + non-null CameraCell hit: 17,748
inside=True frames, 0 branch=indoor decisions. Root cause: RR4 wired
BuildingLoader.Build with the per-frame drainedCells dict — cells that
streamed in on earlier frames (the common case, since cells arrive
asynchronously over many frames after the landblock-info completion)
were not in drainedCells, so the BFS short-circuited and the registry's
EnvCellIds set was systematically incomplete. Cells loaded ahead of
lbInfo arrival never got their BuildingId stamped.

Fix has two parts:

1. CellVisibility.AllLoadedCells — new public IReadOnlyDictionary
   exposing the existing private _cellLookup. BuildingLoader.Build at
   landblock-info-arrival now walks the full cell set, not just this
   frame's drain.

2. _pendingCells drain loop — late-stamps BuildingId on each arriving
   cell if its landblock's BuildingRegistry already exists. Covers cells
   that arrive AFTER the registry-build pass.

Together these handle all four timing cases:
  - Cells loaded before lbInfo arrives  → stamped in BuildingLoader.Build
  - Cells loaded with lbInfo (same frame) → stamped in BuildingLoader.Build
  - Cells loaded after lbInfo arrives    → stamped in drain loop
  - lbInfo never arrives (LB has no info) → registry never built, cells
                                            stay at BuildingId == null
                                            (intended — flow through outdoor
                                            render path)

Probe data from the failed gate launch confirmed cell 0xA9B40150
(cottage idx=6 cellar from the #98 saga) was reached as the camera cell
with visN=16 visible neighbours, but BuildingId stayed null. This fix
gets the indoor branch fired in that scenario.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/CellVisibility.cs

@@ -176,6 +176,18 @@ public sealed class CellVisibility
     /// <summary>Full-ID lookup for O(1) neighbour resolution during BFS.</summary>
     private readonly Dictionary<uint, LoadedCell> _cellLookup = new();
 
+    /// <summary>
+    /// Phase A8 RR7.1 (2026-05-27): read-only view of every loaded cell, keyed
+    /// by full 32-bit cell id. Used by <see cref="Wb.BuildingLoader"/> at
+    /// landblock-info-arrival time so its BFS can reach cells that streamed
+    /// in on earlier frames (not just the per-frame drain). Without this view
+    /// the registry's <c>EnvCellIds</c> set was systematically short, leaving
+    /// <see cref="LoadedCell.BuildingId"/> unset on the cells the camera
+    /// actually enters — which silently routed indoor frames through the
+    /// outdoor branch.
+    /// </summary>
+    public IReadOnlyDictionary<uint, LoadedCell> AllLoadedCells => _cellLookup;
+
     /// <summary>The cell the camera was in during the last <see cref="ComputeVisibility"/> call.</summary>
     private LoadedCell? _lastCameraCell;