feat(phys): A6.P3 #98 Step 2 — cell-dump probe + roundtrip test

Step 2 of the apparatus plan at
C:\Users\erikn\.claude\plans\i-did-some-work-sharded-acorn.md. Adds a
one-shot cell-dump probe so the issue #98 replay harness can load real
cellar / cottage geometry as JSON fixtures, eliminating live-client
iteration from every fix attempt.

Probe gate:
  ACDREAM_DUMP_CELLS=0xA9B40143,0xA9B40146,0xA9B40147
  ACDREAM_DUMP_CELLS_DIR=tests/AcDream.Core.Tests/Fixtures/issue98 (default)

When set, the first time PhysicsDataCache.CacheCellStruct sees a matching
envCellId, it serializes the resulting CellPhysics to
<dir>/0x<cellid>.json and prints one [cell-dump] line. Zero cost when
unset (gate is a static-readonly IReadOnlySet<uint>.Count check).

DTOs (CellDump.cs):
- CellDump: top-level record holding cell id, WorldTransform,
  InverseWorldTransform, resolved polygons, portal polygons, portal
  infos, visible cell ids.
- PolygonDump / PortalDump / Vector3Dto / PlaneDto / Matrix4x4Dto:
  System.Text.Json-friendly records with explicit From / To converters.

What is intentionally NOT dumped: the DAT-native PhysicsBSPTree and
CellBSPTree trees. The replay harness drives the leaf-level walkable
predicates (WalkableHitsSphere, FindCrossedEdge, PolygonHitsSpherePrecise)
directly on the resolved polygon list, which is enough to expose the
issue #98 rejection (poly 0x0004 in 0xA9B40143 reports
insideEdges=False / overlapsSphere=False at the failing-frame sphere).
If a future replay needs BSP traversal we can extend the DTO + Hydrate
together without breaking fixtures.

Tests (CellDumpRoundTripTests):
- Capture → Hydrate preserves WorldTransform / InverseWorldTransform /
  every polygon's plane + vertices + NumPoints + SidesType.
- Capture → Hydrate preserves portal list + visible cell ids.
- Write to disk → Read back → Hydrate preserves content.
- Hydrate leaves BSP / CellBSP null by design (replay uses leaf-level
  predicates).

Verification:
- dotnet build: green, 0 errors.
- dotnet test: 1160 passed + 8 pre-existing failed (was 1156 + 8 before
  this commit; +4 from CellDumpRoundTripTests). Same 8 pre-existing
  failures, no new regressions.

Next: capture the three cells from the live client (Step 2 acceptance),
then build the replay harness against the fixtures (Step 3).

src/AcDream.Core/Physics/PhysicsDataCache.cs

@@ -164,7 +164,7 @@ public sealed class PhysicsDataCache
                 visibleCellIds.Add(lbPrefix | lowId);
         }
 
-        _cellStruct[envCellId] = new CellPhysics
+        var cellPhysics = new CellPhysics
         {
             BSP = cellStruct.PhysicsBSP,
             PhysicsPolygons = cellStruct.PhysicsPolygons,
@@ -178,6 +178,27 @@ public sealed class PhysicsDataCache
             PortalPolygons = portalPolygons,
             VisibleCellIds = visibleCellIds,
         };
+        _cellStruct[envCellId] = cellPhysics;
+
+        if (PhysicsDiagnostics.ProbeDumpCellsEnabled
+            && PhysicsDiagnostics.ProbeDumpCellIds.Contains(envCellId))
+        {
+            try
+            {
+                var dump = CellDumpSerializer.Capture(envCellId, cellPhysics);
+                var path = System.IO.Path.Combine(
+                    PhysicsDiagnostics.ProbeDumpCellsPath,
+                    System.FormattableString.Invariant($"0x{envCellId:X8}.json"));
+                CellDumpSerializer.Write(dump, path);
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[cell-dump] wrote 0x{envCellId:X8} polys={dump.ResolvedPolygons.Count} portals={dump.Portals.Count} → {path}"));
+            }
+            catch (Exception ex)
+            {
+                Console.WriteLine(System.FormattableString.Invariant(
+                    $"[cell-dump] FAILED to dump 0x{envCellId:X8}: {ex.GetType().Name}: {ex.Message}"));
+            }
+        }
 
         if (PhysicsDiagnostics.ProbeCellCacheEnabled)
         {