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

Tools/VoxelForgeTest.ps1

@@ -989,6 +989,7 @@ function Get-CollisionGateStressMetrics([string]$LogPath) {
     $result = [regex]::Match($line, 'result=(?<v>PASS|FAIL)')
     if ($result.Success -and $result.Groups['v'].Value -eq 'PASS') { $metrics.status = 'passed' }
     foreach ($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',
                         'support_first_check_to_submit_s',