test(gate): stress repro judges terrain penetration -- gate on 10/10 clear, gate off 3/3 falls through

The old criteria (travel distance) could not tell a working gate from a broken one: every run,
gate on or off, ended at (455.7, 0, -58.1). That spot is measured air ~1.2 m above a real floor
(density zero-crossing at world Z ~ -275.7 cm), so it is a terrain trap, not a fall-through.

The stress test now drops the pawn fast onto an unready support tile (0,0,-1) and samples the
density at the capsule feet every frame, sweeping at <= half a voxel between positions. PASS =
no measured clearance below -5 cm.

Owner config (radius 3, max level 6, distance 768, sheets off):
- gate on: 10/10 PASS, min clearance +120.5..+120.9 cm, one hold of 0.43-0.47 s each,
  support submitted in ~0.7 ms, collision ready in 0.43-0.47 s
- gate off: 3/3 FAIL terrain_penetration, clearance -5.7..-8.1 cm
Edit-ghost, clipmap no-overlap sweep: pass. Test-harness code only; no production path changed.
This validates a49725d.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

CODEMAP.md

@@ -114,7 +114,7 @@ remains `0`, and lateral regions remain gated off.
 |------|------|
 | `../../VoxelForge.uplugin` | Plugin manifest. Runtime module `VoxelForge` plus the Editor-only `VoxelForgeEditor` commandlet module. Beta. |
 | `VoxelForge.Build.cs` | Deps: Core, CoreUObject, Engine, **GameplayTags**, **RealtimeMeshComponent**. |
-| `Tools/VoxelForgeTest.ps1` | The single guarded test entry point. `-Scenario canonical|owner|probe|perf|tests|parity|horizon|horizon-render|determinism`, optional `-Build`, `-Assets owner|default`, `-Cvars`, absolute `-Out`, `-Label`, `-HorizonLevel`, `-HorizonRenderDistanceChunks`, and `-SharedDDC`; stages the host/assets, isolates user/DDC/config state, serializes one Unreal launch at a time, enforces the 8 GiB/30-minute guards, and emits compact `result.json`/`summary.txt` plus full logs. Horizon traces report desired/loaded tiles, resident stream geometry, worker seconds, steady wall time, and peak private memory; `horizon-render` captures the owner view; `determinism` compares per-tile hashes across the default CRT path and `-voxel.CrtFma3=0`. |
+| `Tools/VoxelForgeTest.ps1` | The single guarded test entry point. `-Scenario canonical|owner|probe|perf|tests|surface-fall|crossing|gate-stress|parity|horizon|horizon-render|determinism`, optional `-Build`, `-Assets owner|default`, `-Cvars`, absolute `-Out`, `-Label`, `-HorizonLevel`, `-HorizonRenderDistanceChunks`, and `-SharedDDC`; stages the host/assets, isolates user/DDC/config state, serializes one Unreal launch at a time, enforces the 8 GiB/30-minute guards, and emits compact `result.json`/`summary.txt` plus full logs. Horizon traces report desired/loaded tiles, resident stream geometry, worker seconds, steady wall time, and peak private memory; `horizon-render` captures the owner view; `determinism` compares per-tile hashes across the default CRT path and `-voxel.CrtFma3=0`. |
 | `Public/VoxelForgeModule.h` / `Private/VoxelForgeModule.cpp` | `FVoxelForgeModule` boilerplate (Startup/Shutdown just log). |
 | `Source/VoxelForgeEditor/VoxelForgeEditor.Build.cs` / `Private/VoxelForgeExploreCommandlet.*` | Editor/commandlet-only `-run=VoxelForgeExplore`; owns bounded render/`VF_` walk/OBJ export orchestration and is not linked by Game/Shipping. |
 | `Public/VoxelGeometryHash.h` | Header-only geometry digest shared by commandlet export and runtime tile-hash dump; hashes mesh counts and the vertex/normal/UV/color/triangle arrays with the historical CRC order. |
@@ -311,6 +311,7 @@ casse cette arithmetique *structurellement* — `BeginPlay` loggue une **Error**
 | `RecordTileHash` / `WriteTileHashDump` | — | Headless `-voxel.TileHashDump=<absolute path>` parity instrument. Records the exact `FVoxelMeshData` used by the streaming result, including empty/classified tiles, then writes sorted absolute tile keys, LOD step/cells, content-band limits, settings digest context, and the shared `VoxelGeometryHash` at `EndPlay`. |
 | `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. |
+| `AdvanceHeadlessCollisionGateStressTest` / `ObserveHeadlessCollisionGateStressTest` / `MaybeFinishHeadlessCollisionGateStressTest` | — | Explicit `-voxel.TestCollisionGateStress=1` harness only. The `GetDensityAt` column at capsule feet defines the local air/solid surface; each frame records actor position, velocity, direct foot tile and gate support readiness, gate state, signed feet clearance, and a ≤0.5-voxel sweep between positions. PASS requires no measured clearance below −5 cm; support submission/readiness, travel, and holds are diagnostics. Result logs include the terminal density column and the first crossing below spawn height. |
 | `BuildDesiredTiles` | ~722 | Builds `DesiredSorted`+`DesiredStamped` (player clipmap shells + F18 sheet ring) then `AddAnchorDesiredTiles()` before the leaver sweep; it orders from the true local pawn position and builds `CriticalDesiredTiles` for the occupied/support and heading tiles. |
 | `AddAnchorDesiredTiles` | — | **§9.3 multi-anchor:** folds each `FVoxelStreamingAnchor`'s thin level-0 box (`XYRadiusChunks`/`ZBelowChunks`/`ZAboveChunks`) into the SAME desired set (deduped by stamp) so AI/remote players keep collision loaded around them; delta cull releases them on move/unregister. **§9.4:** a tile only a CollisionOnly anchor wants (clipmap didn't stamp it) → `CollisionOnlyTiles` → hidden at apply. Zero cost when no anchors. |
 | `ReconcileAnchorTileVisibility` | — | **§9.4:** after each rebuild, toggle `SetVisibility` on ALREADY-LOADED tiles that flipped render↔collision-only (diff `CollisionOnlyTiles` vs prev — bounded, no O(loaded) scan; unhide only if still desired). No-op without CollisionOnly anchors. |