perf(generation): size the room cache to the tile; spatial index; memo-key guard

Coarse tiles thrashed the per-worker 4-chunk room-cache window: 633 BuildChunkCache calls per
LOD3 tile and 1,062 per LOD4 tile. This round was started by one Codex session and finished by a
second, after the orchestrating session died at 06:06 and corrupted the first session's record.
The second session reviewed and built its predecessor's half-finished tree.

- voxel.TileCacheWindow (default 1): the room cache covers the requesting tile's footprint plus a
  fixed collect margin. It is set with TGuardValue around GenerateMesh and classify, so it never
  leaks to other work on the thread. Tile windows are capped at 64 chunks per side;
  unsupported/non-finite params fall back to the legacy window.
- Window invariance, argued by Codex: graph decisions depend on at most two bounded
  MaxTunnelLength hops, and primitives are culled only once their complete bounds are known.
  So the collect margin 2*MaxTunnelLength + max(room reach, pair/chain/tube reach) does not grow
  with the window. Caveat: oversized authored terrain-op radii are not yet in the envelope check.
- PrepareTunnelCoreCache reuses the graph, rebuilding only the exact support-column table for a
  crossed chunk.
- FChunkSDFSpatialIndex: ordered XY buckets over room/join/tunnel bounds; rejection stays exact
  and within-bucket order is preserved. Only 9-13% of candidates reach exact evaluation.
- static_asserts in VoxelStrateTypes.h: VF_STRATE_PARAM_FIELDS lists 83 fields, and their byte sum
  + 8 B padding == sizeof(FStrateGenerationParams). This closes the memo key's known gap (it
  catches omissions and size changes, not same-size replacements or padding-hidden fields).

Verified (game path, two runs each, sampler off):
- field exact: 841/841 per-tile triangle identity, 669,834 triangles; export OBJ SHA-256
  unchanged (b3e5f4c3..9b377); capability 20,830 / 10,909, connected
- cache builds: fused 139,187 -> 1,019 (-99.3%), op-stack 60,598 -> 879
- worker CPU: static 58.4 -> 40.8 s (-30%), moving 138.4 -> 121.2 s (-12%)

REGRESSION, stated plainly: with the switch on, LOD0 gets slower. Generation p50 is ~9% slower,
static 75 -> 82 ms and moving 71 -> 77 ms; request-to-ready p50/p95 is ~5-6% slower. This is
consistent across both runs of both cases. The thrash only lived at LOD2+ (LOD0 had 1.66 builds
per tile), so the next round gates the tile window by LOD and must bring LOD0 back to switch-off
speed while keeping the CPU gain.

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

CODEMAP.md

@@ -302,6 +302,7 @@ casse cette arithmetique *structurellement* — `BeginPlay` loggue une **Error**
 | `ProcessPendingChunks` | 301 | Drains ProcessQueue under per-frame budget; reads the per-request cancellation token before removing its map entry, drops obsolete/aborted results, then applies each live result via `ApplyTileResult`. |
 | `ApplyTileResult` | — | **Shared game-thread apply** for one `FChunkResult` (async drain + sync carve): discards stale generation epochs and keys no longer desired, marks loaded, ingests capture, EMPTY releases the tile's existing component (a re-gen can flip content→empty on band change — old geometry must not linger), else `ApplyMeshToTile`. Returns true iff a visible mesh uploaded (counts the budget). Doesn't touch `PendingTiles` (caller's). |
 | `FVoxelStackSampler` / `FScopedVoxelStackRegistration` (`VoxelStackSampler.h/.cpp`) | — | **Diagnostic stack-sampling profiler (2026-09-13), off by default.** A sampler thread captures registered generation workers' stacks (`CaptureThreadStackBackTrace`) every N µs and tags each with the tile's LOD. It symbolises after the run (inline frames too) into CSV plus a summary under `Saved/`. Registration is per tile TASK in `LoadTile` (and the export's tasks), never per sample; disabled costs one relaxed atomic load. Game `-voxel.SampleStacks=<us>`, commandlet `-samplestacks[=us]`. ~4-5% overhead at 1 ms, so trust large shares, not single-digit deltas. Report: `PERF-SAMPLED.md`. |
+| Room-cache performance (2026-09-13): `VF_FindPlayerFitPointForRoomMemoized` / `GPlayerFitMemo` (`VoxelCaveMorphology.cpp`), `voxel.TileCacheWindow` + `VoxelGenLOD::TileOriginVoxels/TileCellsPerAxis` (`VoxelGenerator.h`), `FChunkSDFSpatialIndex` | — | **Player-fit memo:** exact 97-word key, process-wide, `voxel.PlayerFitMemo`. **Tile cache window:** the room cache covers the requesting tile's footprint plus a fixed collect margin. It is set via `TGuardValue` in `GenerateMesh`/classify, and is window-invariant by construction (the margin does not grow with the tile). **Spatial index:** ordered XY buckets over cache primitives, exact rejection. The `static_assert` in `VoxelStrateTypes.h` ties `VF_STRATE_PARAM_FIELDS` to `FStrateGenerationParams` (83 fields + 8 B padding). |
 | `GenerateTileResult` | — | **Shared worker-side gen** for one tile (async `LoadTile` task + sync `SyncRemeshTile`): [ClassifyTile (T1.d) + exact validation, only when `voxel.OuterClassifierMode=1`; **default 0 since 2026-09-13**] → `GenerateMesh`/`GenerateSheetMesh` → `BuildTileStreamSet`. Reads Generator/Mesher only → safe on a worker or the game thread; fills `FChunkResult`, no enqueue. |
 | `SyncRemeshTile` | — | **INSTANT DIG**: level-0 same-frame re-mesh on the game thread (`GenerateTileResult` + `ApplyTileResult` inline, Cells=CHUNK_SIZE/Step=1 + strate band, no capture). Used for the tile under the brush centre so a carve is visible THIS frame; one full-res gen on the game thread. |
 | `UpdateChunksAroundPosition` | 362 | Builds desired set, cancels pending keys that left it, sorts from the **true pawn position**, submits the occupied/support LOD0 tile and the next tile along the pawn heading as an absolute `BackgroundHigh` prefix, then loads/unloads and handles LOD changes. **Delta cull**: `BuildDesiredTiles` returns the LEAVERS (stamped `DesiredStamped` map, one sweep) — only those + `TransitionHold` are considered per crossing, not every loaded tile. `BuildDesiredTiles` also applies `RenderDistanceChunks`: the outermost shell widens to cover the distance — as level-MaxClipLevel MC tiles, or (F18 `bFarSheetRing`) as a SHEET ring at level MaxClipLevel+span (covered-check vs the MaxClipLevel box; `VF_OuterShell` shared with `IsTileInClipRange` so the cull sees the same horizon), dz pre-clamped to the vertical band. **§9.3 anchors:** also prunes dead `StreamingAnchors` + detects their chunk crossings → rebuilds the desired set when an anchor moves/(un)registers (`bAnchorsMoved`/`bForceDesiredRebuild`), same cadence as player movement. §8.10. |