measure(perf): in-process stack sampler; the game spends ~3/4 of worker CPU on room player-fit

A diagnostic stack-sampling profiler, off by default. A sampler thread captures the stacks of
registered generation workers with FPlatformStackWalk::CaptureThreadStackBackTrace every N us and
tags each sample with the tile's LOD. Symbolisation (inline frames included) happens after the
run, into a bounded CSV plus a summary. Registration is per tile task, never per sample; when the
sampler is off it costs one relaxed atomic load. Game: -voxel.SampleStacks=<us>. Commandlet:
-samplestacks[=us]. The streaming latency line now also prints applied tile, visible tile and
triangle counts.

Instrument checks (Codex, PERF-SAMPLED.md):
- a planted 20 ms busy loop was found at 10.5% inclusive, the expected share, then removed
- closure: 193k static / 251k moving samples, 0 capture failures, <0.75% wait frames,
  leaf resolved for 82-84%
- static game output identical with the sampler on or off: 841 tiles, 447 visible, 669,834
  triangles
- overhead at 1 ms: +4.7% static worker CPU; moving worker CPU within noise, moving timing +4-5%.
  So trust large shares, not single-digit deltas.

The finding, on the game path (the owner's instruction: profile the game, not the export):

                                all-LOD inclusive   static   moving
  BuildChunkCache                                   79.5%    80.5%
  VF_FindPlayerFitPointForRoom                      73.4%    74.0%

About three quarters of in-game worker CPU goes to player-fit searches for rooms. They are
recomputed on every per-worker room-cache rebuild, and coarse tiles span many cache windows. At
LOD0 the costs are the cave warp (3 Perlin per sample), the swept tunnel evaluated both in the
SDF and in EvaluateTunnelCoreWorld (~12% inclusive each, duplication confirmed), and the passage
structural tail (~11-14%). Candidate AABB rejection is cheap (1.5-2.2%). The 74-field params copy
does not show up as a separate cost.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VZS94UH8bSsBeg5sZaxe2Y

CODEMAP.md

@@ -301,6 +301,7 @@ casse cette arithmetique *structurellement* — `BeginPlay` loggue une **Error**
 | `GetPlayerPosition` | 231 | Pawn position or zero. (`GetLODForChunk`/`LODToStep`/`IsChunkInRange` removed — dead since the clipmap.) |
 | `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`. |
 | `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. |