perf(review): gate never deadlocks, collision radius, 20x faster digs

From the outside performance review (items 1-4; item 5 prototyped and reverted).

1. Collision gate: a support request outside the stamped desired set was cancelled by the rebuild
   it forced, and the gate held the pawn with gravity 0. Baseline deep-drop (floor at (0,0,-7),
   outside the aligned L0 ring): 1 hold of 30.0 s, never released. Fix: the target and 3 tiles
   below are retained through rebuilds at CollisionCritical; hold only when the swept capsule is
   about to enter pending support; release restores movement mode, gravity and downward speed.
   After: 3 holds, 2.21 s total, 112.8 m horizontal progress; gate off still penetrates (-7.4 cm).
2. Suspected cleanup regression (out-of-layout stacks): not real. 44 paired boundary tiles,
   30.9 ms vs 32.6 ms at b4ac996, hashes 44/44 identical. No change.
3. PawnCollisionRadiusChunks (new, default 1): L0 tiles outside the collision set stay visible
   without Chaos cooks; anchors and the gate's support column are unioned in. Cook requests
   ~217/min -> ~30/min (-86%). Critical support ready time did NOT improve (9.5 s -> 10.6 s in the
   deep-drop fixture; the wait is the cook itself, ~7.4 s cook-to-ready there).
4. Dig: exact pre-edit L0 density cached (one 35^3 grid, 171.5 KB), carve replays current mods on
   it. Sync remesh 99.4 ms -> 4.7 ms. 16/16 random carves hash-identical to full regeneration.
5. Parallel grid fill for the critical tile: 16/16 hash parity, but the waited-on tile was already
   meshed; the wait was collision cooking. Reverted.

Review G answered: M_Test1 still ray-marches VolTex0 (custom HLSL mini-sun shadow), so the density
volume is live. Known and left alone: outside the strate layout the fallback density can be solid
while the gate treats those tiles as air.
Tests: VoxelForgeCollisionDesiredTilesTest, VoxelForgePreEditDensityCacheTest; no-overlap, edit
ghost, descent corridor pass.

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

Source/VoxelForge/Public/VoxelWorld.h

@@ -149,6 +149,9 @@ struct FChunkResult
     // it to UVoxelDensityVolume::IngestTileCapture so the density clipmap reuses the mesher's samples
     // instead of re-sampling. Moved (not copied) through the MPSC queue. See UVoxelDensityVolume.
     TArray<uint8> CaptureGrid;
+    // Exact pre-edit level-0 lattice for the bounded local-player dig cache. Sampled on the worker
+    // with the tile's normal generator context and moved through the result queue.
+    FVoxelSharedDensityGrid PreEditDensityGrid;
 };
 
 /** One worker-produced mesh identity emitted only by the explicit parity harness. */
@@ -550,7 +553,7 @@ private:
     bool MeasureHeadlessCollisionGateStressTerrain(
         const FVector& WorldPosition, float CapsuleHalfHeightCm,
         float& OutFeetDensity, float& OutCenterDensity,
-        float& OutSurfaceLocalVoxelZ, float& OutClearanceCm) const;
+        float& OutSurfaceLocalVoxelZ, float& OutClearanceCm);
     void AdvanceHeadlessSurfaceFallTest(FVector& InOutPlayerPosition,
                                         FVector& InOutPlayerHeading, APawn* PlayerPawn);
     void InitializeHeadlessStrateCrossingTest();
@@ -769,7 +772,9 @@ public:
                             int32 Step, int32 Cells, uint32 Epoch, bool bWantCapture,
                             int32 BandVoxLo, int32 BandVoxHi, int32 BandChunkLo, int32 BandChunkHi,
                             FChunkResult& Result,
-                            const std::atomic<bool>* ObsoleteFlag = nullptr);
+                            const std::atomic<bool>* ObsoleteFlag = nullptr,
+                            bool bWantPreEditDensityGrid = false,
+                            const FVoxelSharedDensityGrid* SharedDensityOverride = nullptr);
 
     /**
      * Game-thread apply for one gen result (shared by ProcessPendingChunks + SyncRemeshTile):
@@ -859,6 +864,7 @@ public:
                               FVoxelTileKey& OutSupportTile) const;
 
     bool IsTileCollisionReady(const FVoxelTileKey& Tile) const;
+    bool IsTileCollisionDesired(const FVoxelTileKey& Tile) const;
     bool IsCollisionTileUnavailable(const FVoxelTileKey& Tile,
                                     const TCHAR*& OutReason) const;
     bool IsCollisionReadyFromSupportTile(const FVoxelTileKey& CandidateTile,
@@ -872,10 +878,10 @@ public:
                                               FVoxelTileKey* OutPendingTile = nullptr) const;
     void RequestCollisionGateSupportTile(const FVoxelTileKey& SupportTile);
 
-    /** Cancel only downward pawn motion while the tile beneath its feet is unresolved. Horizontal
-     *  input and movement remain live. The world tick is installed as the pawn prerequisite in the
-     *  implementation (and directly on its movement component) so this check runs before
-     *  CharacterMovementComponent physics. */
+    /** Hold vertical character motion in gravity-free MOVE_Flying while a swept pending support tile
+     *  intersects the capsule, preserving horizontal movement until collision is ready. Restore the
+     *  previous mode, gravity, and downward speed on release. The world tick is a prerequisite of the
+     *  pawn and movement component so the gate check runs before movement physics. */
     void UpdatePawnCollisionGate(APawn* Pawn, const FVector& PlayerPosition, float DeltaTime);
     void EngagePawnCollisionGate(APawn* Pawn);
     void ReleasePawnCollisionGate();
@@ -951,6 +957,9 @@ public:
 
     // Player's level-0 tile coord (= chunk coord). The desired set is rebuilt when this changes.
     FIntVector CurrentCenterChunk = FIntVector::ZeroValue;
+    // Exact unedited L0 lattices retained around the player for same-frame carve remeshes.
+    // The radius helper bounds this to 27 × 35³ floats (4.42 MiB).
+    TMap<FVoxelTileKey, FVoxelSharedDensityGrid> PreEditDensityCache;
 
     // Non-player streaming anchors (AI now, remote players later — ARCHITECTURE §9.3). BuildDesiredTiles
     // folds each anchor's small level-0 box into the desired set (same DesiredStamped machinery → the
@@ -960,6 +969,7 @@ public:
     // Adds each anchor's level-0 tiles to DesiredSorted/DesiredStamped (dedup vs the player clipmap)
     // and records the ones ONLY a CollisionOnly anchor wants in CollisionOnlyTiles.
     void AddAnchorDesiredTiles();
+    void ReconcileCollisionDesiredTiles();
     // Forces a desired-set rebuild next Tick even if neither the player nor an anchor crossed a tile
     // boundary (set by UnregisterStreamingAnchor so a removed anchor's tiles get culled).
     bool bForceDesiredRebuild = false;
@@ -971,6 +981,9 @@ public:
     // without an O(loaded) scan. Both empty when there are no CollisionOnly anchors → zero cost.
     TSet<FVoxelTileKey> CollisionOnlyTiles;
     TSet<FVoxelTileKey> PrevCollisionOnlyTiles;
+    // Independent from DesiredSorted: rendered LOD0 tiles outside the pawn radius remain visible
+    // without a Chaos body. Pawn radius, anchor boxes, and collision-gate support are unioned here.
+    TSet<FVoxelTileKey> CollisionDesiredTiles;
     // A short collision-only column below an unresolved player support tile. Keeping these keys
     // stamped prevents a critical request outside the render clipmap from being canceled on rebuild.
     TSet<FVoxelTileKey> CollisionGateDesiredTiles;
@@ -1103,14 +1116,18 @@ public:
     bool bHeadlessCollisionGateStressTestFailed = false;
     bool bHeadlessCollisionGateStressTestExitRequested = false;
     bool bHeadlessCollisionGateStressTestGateWasEngaged = false;
+    bool bHeadlessCollisionGateStressTestHorizontalGateProbeApplied = false;
     bool bHeadlessCollisionGateStressTestFullStop = false;
     bool bHeadlessCollisionGateStressTestStartHeightCrossed = false;
     bool bHeadlessCollisionGateStressTestTerrainViolation = false;
     bool bHeadlessCollisionGateStressTestTerrainMeasurementMissing = false;
     bool bHeadlessCollisionGateStressTestLanded = false;
     bool bHeadlessCollisionGateStressTestLaunchSupportTimingCaptured = false;
+    bool bHeadlessCollisionGateStressTestSettingsReported = false;
     bool bHeadlessCollisionGateStressDeepDrop = false;
     bool bHeadlessCollisionGateStressDeepDropPrepared = false;
+    bool bHeadlessCollisionGateStressDeepDropPrewarmReported = false;
+    bool bHeadlessCollisionGateStressDeepDropSurfaceCached = false;
     float HeadlessCollisionGateStressTestSpeedCmPerSecond = 6000.0f;
     float HeadlessCollisionGateStressTestSpawnHeightVoxels = 10.0f;
     float HeadlessCollisionGateStressTestDurationSeconds = 2.0f;
@@ -1139,6 +1156,8 @@ public:
     FVector HeadlessCollisionGateStressTestLastActualPosition = FVector::ZeroVector;
     FVector HeadlessCollisionGateStressTestStartPosition = FVector::ZeroVector;
     FVector HeadlessCollisionGateStressTestMinClearancePosition = FVector::ZeroVector;
+    FVector2D HeadlessCollisionGateStressDeepDropSurfaceXYVox = FVector2D::ZeroVector;
+    float HeadlessCollisionGateStressDeepDropSurfaceZVox = 0.0f;
     FVoxelTileKey HeadlessCollisionGateStressTestLaunchSupportTile;
     FString HeadlessCollisionGateStressTestFailureReason;
 
@@ -1235,6 +1254,9 @@ public:
     bool bCollisionGateEnabled = true;
     float SavedCharacterGravityScale = 1.0f;
     bool bSavedCharacterGravityScale = false;
+    uint8 SavedCharacterMovementMode = 0;
+    uint8 SavedCharacterCustomMovementMode = 0;
+    bool bSavedCharacterMovementMode = false;
     float SavedGateVerticalVelocity = 0.0f;
     bool bSavedGateVerticalVelocity = false;
     bool bPawnCollisionGateEngaged = false;