wip(gate): support column kept through rebuilds, hold only on swept contact -- NEVER COMPILED OR RUN

Stopped mid-round: the PC ran out of memory (another app held 11 GB), new processes failed to start
(0xC0000142), and the round ended without verifying anything after the edit.

Confirmed in code (performance review finding 1): a support request outside the stamped desired set
forces a rebuild that cancels it, the gate asks for one unresolved tile at a time, and it holds
before the pending tile is anywhere near the pawn's next step.

Uncompiled edits: a short collision-only support column retained through desired rebuilds and
prioritised; hold only when the swept capsule is about to enter a pending tile; downward speed
restored on release; opt-in deep-drop fixture (carved shaft) in the stress harness.

Evidence so far: shallow 10-voxel fall, gate on, before the edit: 1 hold of 1.51 s, clearance
+120.6 cm. The 80/160-voxel attempts spawned OUTSIDE the strate layout (support unavailable,
zero holds) and failed the penetration check at -11.1 cm: not the reviewed loop, but worth a look.
Compile risks: GetGravityZ(), TSet pruning in the rebuild, the fixture's carve/log arguments.

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

CODEMAP.md

@@ -308,8 +308,10 @@ 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` | 6870 | 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 (`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` | 6482 | Builds `DesiredSorted`+`DesiredStamped` (player clipmap shells) then `AddAnchorDesiredTiles()` before the leaver sweep; it orders from the true local pawn position and builds `CriticalDesiredTiles` for the occupied/support and heading tiles. Also rebuilds `DesiredTransitionDescendants` (ancestor → desired descendants) for the reconcile pass. |
+| `AdvanceHeadlessCollisionGateStressTest` / `ObserveHeadlessCollisionGateStressTest` / `MaybeFinishHeadlessCollisionGateStressTest` | — | Explicit `-voxel.TestCollisionGateStress=1` harness only. `voxel.TestCollisionGateStressSpawnHeightVoxels` sets the drop above the measured density surface (default 10 voxels); optional `voxel.TestCollisionGateStressDeepDrop=1` carves a process-local shaft for a fall deeper than the level-0 ring. 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 first crossing below spawn height. |
+| `BuildDesiredTiles` | 6482 | Builds `DesiredSorted`+`DesiredStamped` (player clipmap shells), then adds streaming anchors and the pending player's collision-only support column before the leaver sweep. It orders from the true local pawn position and builds `CriticalDesiredTiles` for the unresolved support target, occupied/support and heading tiles. Also rebuilds `DesiredTransitionDescendants` (ancestor → desired descendants) for the reconcile pass. |
+| `CollisionGateDesiredTiles` / `RequestCollisionGateSupportTile` | 5062 | Retains an unresolved support target and up to three level-0 tiles below it across desired-set rebuilds. The additional keys are hidden collision-only tiles, queued ahead of ordinary streaming, and pruned once resolved or outside the active column. |
+| `IsPawnCapsuleOverlappingPendingFloor` / `UpdatePawnCollisionGate` | 4987 / 5195 | Uses the current-to-next-step swept capsule bounds to start a hold only when the pending support tile is about to intersect the pawn. A hold preserves horizontal input and restores downward speed after collision becomes ready. |
 | `VoxelClipmapDesiredTiles::Build` / `AlignedRingBounds` (`Private/VoxelClipmapDesiredTiles.h`) | — | The pure clipmap selector. **Every finer ring snaps outward to a child-pair boundary** (`AlignedRingBounds`, alignment 2; the outermost ring needs none), so a coarser tile is wholly inside or wholly outside the finer ring: **no two desired tiles of different levels overlap** (an unaligned 7-wide odd ring leaves one half-covered coarse tile per axis side, drawn over the fine one: 635 pairs at the owner config). Cost: LOD0 ring 343 → 512 tiles. Guarded by `VoxelForge.Streaming.ClipmapDesiredTilesNoOverlapCoverage`. Anchor tiles appended later are outside this guarantee. |
 | `ReconcileReadyTransitionVisibility` | — | Visibility-only pass while streaming is unsettled: hides a retired tile once all desired tiles over its footprint are loaded, reveals new tiles once no visible retired tile overlaps them. **Indexed** (dyadic ancestors + `DesiredTransitionDescendants`, visible retired tiles collected once per pass): moving p95 32 ms → 0.31 ms. `voxel.MeasureReadyTransitionVisibility=1` logs per-frame samples. |
 | `RequestCollisionGateSupportTile` / `UpdatePawnCollisionGate` | — | The gate submits the pawn's level-0 support tile **immediately** as `CollisionCritical`, even before the first desired set exists (the desired rebuild then adopts the same pending key). The gate holds downward Z only, with **no timeout** (a 2 s release would fire at every startup, while the game thread applies nothing for ~2.2 s). |