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

Tools/VoxelForgeTest.ps1

@@ -216,6 +216,40 @@ function Assert-NoForeignUnreal([string]$When) {
     }
 }
 
+if (-not ('VoxelForgeTestMemoryStatus' -as [type])) {
+    Add-Type -TypeDefinition @'
+using System;
+using System.Runtime.InteropServices;
+public static class VoxelForgeTestMemoryStatus {
+    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
+    public class MEMORYSTATUSEX {
+        public uint dwLength = (uint)Marshal.SizeOf(typeof(MEMORYSTATUSEX));
+        public uint dwMemoryLoad;
+        public ulong ullTotalPhys;
+        public ulong ullAvailPhys;
+        public ulong ullTotalPageFile;
+        public ulong ullAvailPageFile;
+        public ulong ullTotalVirtual;
+        public ulong ullAvailVirtual;
+        public ulong ullAvailExtendedVirtual;
+    }
+    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
+    public static extern bool GlobalMemoryStatusEx([In, Out] MEMORYSTATUSEX lpBuffer);
+}
+'@
+}
+
+function Assert-EnoughFreeMemory([string]$When) {
+    $status = New-Object VoxelForgeTestMemoryStatus+MEMORYSTATUSEX
+    if (-not [VoxelForgeTestMemoryStatus]::GlobalMemoryStatusEx($status)) {
+        throw "Could not query free physical memory before $When; refusing to launch."
+    }
+    $freeGiB = [double]$status.ullAvailPhys / 1GB
+    if ($freeGiB -lt 6.0) {
+        throw ("Only {0:N2} GiB free before {1}; refusing to launch (minimum 6 GiB)." -f $freeGiB, $When)
+    }
+}
+
 function Get-CrashSnapshot {
     $snapshot = [ordered]@{}
     $roots = @(
@@ -330,6 +364,7 @@ function Wait-OwnedProcess($Process, [int]$TimeoutSeconds, [switch]$EnforceMemor
 function Invoke-UnrealLaunch([string]$Kind, [string[]]$Arguments, [string]$LogPath,
                              [string]$StdoutPath, [string]$StderrPath) {
     Assert-NoForeignUnreal "before $Kind launch"
+    Assert-EnoughFreeMemory "$Kind launch"
     Ensure-Directory (Split-Path -Parent $LogPath)
     Ensure-Directory (Split-Path -Parent $StdoutPath)
     Ensure-Directory (Split-Path -Parent $StderrPath)
@@ -399,6 +434,8 @@ function Invoke-UnrealLaunch([string]$Kind, [string[]]$Arguments, [string]$LogPa
 
 function Invoke-BuildProcess([string[]]$Arguments, [string]$LogPath, [string]$ErrorPath,
                              [string]$StdoutPath = $LogPath) {
+    Assert-NoForeignUnreal 'before staged build launch'
+    Assert-EnoughFreeMemory 'staged build launch'
     Ensure-Directory (Split-Path -Parent $LogPath)
     $dotnetHome = Join-Path $RunRoot 'DotnetHome'
     Ensure-Directory $dotnetHome
@@ -991,7 +1028,8 @@ function Get-CollisionGateStressMetrics([string]$LogPath) {
     foreach ($key in @('reason', 'elapsed_s', 'distance_m', 'expected_distance_m', 'speed_cm_s',
                         'spawn_height_vox', 'deep_drop',
                         'gate_holds', 'gate_total_s', 'max_gate_s', 'full_stops',
-                        'full_stop_total_s', 'collision_gate', 'gate_progress_m', 'support_tile',
+                        'full_stop_total_s', 'collision_gate', 'gate_progress_m',
+                        'horizontal_gate_probe', 'support_tile',
                         'support_first_check_to_submit_s',
                         'support_first_check_to_collision_ready_s', 'test_support_tile',
                         'test_support_first_check_to_submit_s',