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/Private/VoxelCollisionDesiredTiles.h

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "CoreMinimal.h"
+#include "VoxelTypes.h"
+
+namespace VoxelCollisionDesiredTiles
+{
+    inline void AddBox(TSet<FVoxelTileKey>& OutTiles, const FIntVector& Center,
+                       int32 XYRadius, int32 ZBelow, int32 ZAbove)
+    {
+        const int32 SafeXYRadius = FMath::Max(0, XYRadius);
+        const int32 SafeZBelow = FMath::Max(0, ZBelow);
+        const int32 SafeZAbove = FMath::Max(0, ZAbove);
+        for (int32 Z = -SafeZBelow; Z <= SafeZAbove; ++Z)
+        {
+            for (int32 Y = -SafeXYRadius; Y <= SafeXYRadius; ++Y)
+            {
+                for (int32 X = -SafeXYRadius; X <= SafeXYRadius; ++X)
+                {
+                    OutTiles.Add(FVoxelTileKey(Center + FIntVector(X, Y, Z), 0));
+                }
+            }
+        }
+    }
+}