perf(generation): gate the tile cache window by LOD; LOD0 back to switch-off speed

2de603c's tile-sized room cache cut worker CPU, but LOD0 generation was ~9% slower. The per-tile
split shows the loss was evaluation (more candidates per sample in a bigger window), not cache
building. Policy, measured per path and LOD:
- fused path: tile window at LOD >= voxel.TileCacheWindowMinLOD (default 3)
- op-stack path: tile window at LOD >= voxel.TileCacheWindowOpStackMinLOD (default 4)
- lower LODs keep the 4-chunk worker window
- voxel.SpatialIndex: -1 auto (off at fused LOD0, on elsewhere), 0 or 1

Proof caveat closed: VF_ComputeTerrainOpEnvelope adds authored terrain-op extents (pits, chimneys,
detail overrides, ...) to the invariance envelope. Ops beyond the proven 256-voxel extent, or
malformed/unloaded ones, force the legacy window, so window invariance holds for every asset.

Sampler fix: the leaf symbol kept pointers into a TMap that FindOrAdd can rehash while later
frames resolve; the leaf is now re-found by program counter after insertion.

Game path, two runs each, sampler off (Codex; checked against its report):
                       LOD0 request p50/p95    generation p50/p95    worker s
  static  optimized    106.3 / 175.9 ms        76.6 / 124.3 ms       39.2
  static  switch-off   106.3 / 173.0           75.9 / 124.1          58.5   (-33%)
  moving  optimized     79.4 / 158.5           72.4 / 123.7         113.7
  moving  switch-off    82.8 / 160.4           73.2 / 125.4         141.9   (-20%)
LOD0 is flat within noise static and slightly better moving; worker CPU keeps most of the gain.
Field exact: 841/841 per-tile triangle identity, export OBJ SHA-256 unchanged (b3e5f4c3..9b377),
capability 20,830 / 10,909.

Caution: compare these numbers only within this round. The machine is in daytime use, so
absolute figures drift between rounds.

Current LOD0 map (sampler, moving, inclusive): tunnel SDF 12.5% + tunnel core 12.3% (the same
tunnel in two coordinate spaces); passage carving 11.1%, structural posts 11.4%, native floor
4.1%, with floor projection at 8.9% inside them; cave warp ~8% exclusive. Development-build-only
overheads: TArray RangeCheck ~6.5%, per-sample FScopedTimer ~2.6%.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TaFdXhTdYMkW1xrV6ApT6Y

Source/VoxelForge/Private/VoxelWorld.cpp

@@ -3849,9 +3849,21 @@ void AVoxelWorld::GenerateTileResult(const FVoxelTileKey& Tile, const FIntVector
                 return bMeasureTile && TileProfileEnd.Counters[Index] >= TileProfileStart.Counters[Index]
                     ? TileProfileEnd.Counters[Index] - TileProfileStart.Counters[Index] : 0;
             };
+            const auto CycleDelta = [&](VoxelDensityProfile::EBucket Bucket) -> uint64
+            {
+                const int32 Index = static_cast<int32>(Bucket);
+                return bMeasureTile && TileProfileEnd.Cycles[Index] >= TileProfileStart.Cycles[Index]
+                    ? TileProfileEnd.Cycles[Index] - TileProfileStart.Cycles[Index] : 0;
+            };
+            const double TileSeconds = FPlatformTime::Seconds() - TileStartSeconds;
+            const double CacheBuildSeconds = FPlatformTime::ToSeconds64(
+                CycleDelta(VoxelDensityProfile::EBucket::RoomGraphBuild));
+            const double EvaluationSeconds = FMath::Max(
+                0.0, TileSeconds - CacheBuildSeconds);
             UE_LOG(LogTemp, Display,
                 TEXT("[VoxelForgeTileProfile] tile=(%d,%d,%d) level=%d step=%d cells=%d sheet=%d aborted=%d empty=%d "
                      "verdict=%d classify=%.6f mesh=%.6f streams=%.6f seconds=%.6f "
+                     "cache_build=%.6f evaluation=%.6f "
                      "refine=%u stack=%u core_samples=%u final_samples=%u core_hits=%u final_hits=%u "
                      "core_leaves=%u final_leaves=%u tail_queries=%u tail_eval=%u "
                      "stack_work=%.6f core_work=%.6f final_work=%.6f tail_work=%.6f "
@@ -3868,7 +3880,7 @@ void AVoxelWorld::GenerateTileResult(const FVoxelTileKey& Tile, const FIntVector
                 Tile.Coord.X, Tile.Coord.Y, Tile.Coord.Z, Tile.Level, Step, Cells,
                 bSheetTile ? 1 : 0, Result.bAborted ? 1 : 0,
                 Result.bEmpty ? 1 : 0, ClassifyVerdict, ClassifySeconds, MeshSeconds, StreamSeconds,
-                FPlatformTime::Seconds() - TileStartSeconds,
+                TileSeconds, CacheBuildSeconds, EvaluationSeconds,
                 ClassifierStats.RefineNodes, ClassifierStats.StackBoxCalls,
                 ClassifierStats.ExactCoreSamples, ClassifierStats.ExactFinalSamples,
                 ClassifierStats.ExactCoreCacheHits, ClassifierStats.ExactFinalCacheHits,