refactor(physics): Phase 2 — code-review polish on BuildingPhysics commit

Five reviewer-flagged items addressed:

- Fix #1: GameWindow building-loop now reuses TerrainSurface.ComputeOutdoorCellId
  instead of re-deriving the row-major cell-index formula. DRY win; no risk
  of the two formulas drifting.
- Fix #2: BuildingPhysics.ExactMatch decoder now references
  DatReaderWriter.Enums.PortalFlags.ExactMatch instead of magic 0x0001.
- Fix #3: ExactMatch XML doc clarified as "reserved per retail's
  CBldPortal::exact_match; not currently consumed by CheckBuildingTransit".
- Fix #4: CheckBuildingTransit docstring now explicitly documents the
  retail divergence — retail's sphere_intersects_cell (radius-aware) vs.
  our PointInsideCellBsp (radius-less). The sphereRadius parameter is
  reserved for the future sphere_intersects_cell port. Practical effect
  noted: entry fires ~sphereRadius (~0.48m) deeper than retail.
- Fix #5: Test method `SphereInsideBuildingPortalDestination_AddsInteriorCell`
  renamed to `BuildingPortalWithUnloadedCellBSP_NoCandidateAdded` — the
  test asserts Empty(candidates), not that the cell is added. Comment
  updated.

Spec: docs/superpowers/specs/2026-05-19-indoor-portal-cell-tracking-design.md
Plan: docs/superpowers/plans/2026-05-19-indoor-portal-cell-tracking.md

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -5741,15 +5741,12 @@ public sealed class GameWindow : IDisposable
                         * System.Numerics.Matrix4x4.CreateTranslation(bldOriginWorld);
 
                     // Derive the outdoor landcell id containing this building.
-                    // Retail's cell index: row-major (gridX * 8 + gridY + 1) within
-                    // the 8×8 grid of 24m cells in a landblock.
-                    int bldGridX = (int)(building.Frame.Origin.X / 24f);
-                    int bldGridY = (int)(building.Frame.Origin.Y / 24f);
-                    if (bldGridX < 0) bldGridX = 0;
-                    if (bldGridX >= 8) bldGridX = 7;
-                    if (bldGridY < 0) bldGridY = 0;
-                    if (bldGridY >= 8) bldGridY = 7;
-                    uint landcellLow = (uint)(bldGridX * 8 + bldGridY + 1);
+                    // Reuse TerrainSurface.ComputeOutdoorCellId rather than
+                    // re-deriving the row-major (gridX * 8 + gridY + 1) formula here.
+                    // Frame.Origin is landblock-relative, same coordinate space as
+                    // ComputeOutdoorCellId expects (local X/Y within the 192m block).
+                    uint landcellLow = terrainSurface.ComputeOutdoorCellId(
+                        building.Frame.Origin.X, building.Frame.Origin.Y);
                     uint landcellId = lbPrefix | landcellLow;
 
                     _physicsDataCache.CacheBuilding(landcellId, bldPortals, buildingTransform);

src/AcDream.Core/Physics/BuildingPhysics.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Numerics;
+using DatReaderWriter.Enums;
 
 namespace AcDream.Core.Physics;
 
@@ -36,6 +37,16 @@ public readonly struct BldPortalInfo
     public ushort OtherPortalId { get; }
     public ushort Flags         { get; }
 
-    /// <summary>Bit 0 of Flags (<c>PortalFlags.ExactMatch = 0x0001</c>).</summary>
-    public bool ExactMatch => (Flags & 0x0001) != 0;
+    /// <summary>
+    /// Bit 0 of <see cref="Flags"/> (<c>DatReaderWriter.Enums.PortalFlags.ExactMatch</c>).
+    ///
+    /// <para>
+    /// Reserved per retail's <c>CBldPortal::exact_match</c>. NOT currently
+    /// consumed by <see cref="CellTransit.CheckBuildingTransit"/> — every
+    /// portal overlap is treated as a valid entry trigger. If a future
+    /// regression surfaces (e.g., a building entered by overlapping a
+    /// non-exact-match portal), wire this into the entry test.
+    /// </para>
+    /// </summary>
+    public bool ExactMatch => (Flags & (ushort)PortalFlags.ExactMatch) != 0;
 }

src/AcDream.Core/Physics/CellTransit.cs

@@ -158,6 +158,23 @@ public static class CellTransit
     /// whether the sphere center is inside it via
     /// <see cref="BSPQuery.PointInsideCellBsp"/>. If so, add the interior
     /// cell to <paramref name="candidates"/>.
+    ///
+    /// <para>
+    /// <b>Retail divergence:</b> retail's <c>check_building_transit</c>
+    /// uses <c>CCellStruct::sphere_intersects_cell</c> (radius-aware
+    /// BSP-vs-sphere test) which fires the moment ANY part of the sphere
+    /// overlaps the destination cell. Our port uses
+    /// <see cref="BSPQuery.PointInsideCellBsp"/> (radius-less, tests only
+    /// the sphere CENTER). Practical effect: entry into a building fires
+    /// when the player's foot-sphere center crosses the destination cell
+    /// boundary — roughly <paramref name="sphereRadius"/> (~0.48m) DEEPER
+    /// into the doorway than retail. If visual verification at the cottage
+    /// door shows a noticeable "late entry" effect (player visually inside
+    /// the building before walls switch from outdoor-stab to indoor-cell),
+    /// port <c>sphere_intersects_cell</c> in a follow-up.
+    /// <paramref name="sphereRadius"/> is plumbed through for that future
+    /// upgrade; currently unused.
+    /// </para>
     /// </summary>
     public static void CheckBuildingTransit(
         PhysicsDataCache cache,