perf(generation): per-tile reach test skips landing and structural posts -- ~12% less worker CPU

The remove-one ablation put landing posts at ~10% and passage structural posts at ~6% of worker
time, though landings exist only near passage ends and the origin. The parked exact-cull round's
per-SAMPLE post bounds had been a net loss. This round decides reach once per TILE (Codex;
reviewed by me).

- VoxelTilePostReach.h: one per-tile reach mask (thread-local byte), set when the tile is set
  up. The origin landing, passage landing SDF/floors, native floors, tunnel air and passage
  structural posts return their input unchanged on tiles no instance can reach. Per sample this
  costs an inline flag read.
- Reach proofs come from each post's geometry (origin rooms, landing SDF and floors, native floor
  band, structural support spheres), not from comments.
- voxel.TilePostReachDebug=1 evaluates the skipped posts anyway and compares the FINAL field;
  voxel.TilePostReachScale=0.5 deliberately shrinks the reach and must fail.

Default build: 694 tiles, landing reachable on 4 (skipped on 690), structural posts skipped on 169.
Soundness: 123,958,673 comparisons, 0 differences; shrunk reach: 13,929 differences, FAIL as
required.

Final DEFAULT build vs the committed-tip DLL (same command line, two interleaved runs each):
  static  worker 34.80 -> 30.52 s (-12%)   LOD0 generation p50/p95 67.0/107.6 -> 55.8/95.8 ms
  moving  worker 75.96 -> 66.66 s (-12%)   LOD0 generation p50/p95 66.3/106.0 -> 56.0/95.8 ms
Field exact: 841/841 keyed tiles vs a fresh committed-tip trace (669,902 triangles, which includes
9260775's gap floors), export b3e5f4c3..9b377, capability 20,830 / 10,909; strate-crossing and
sealed-proof tests pass.
Ledger: all launches passed or failed where designed to; early raw-SDF debug attempts failed
until the comparator was corrected to compare the final field. No crash folders.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TaFdXhTdYMkW1xrV6ApT6Y

Source/VoxelForge/Public/VoxelDensityPrimitives.h

@@ -28,6 +28,7 @@
 #include "VoxelTypes.h"   // SmoothStep01
 #include "VoxelPassageGeometry.h" // shared body-sized landing/tunnel geometry
 #include "VoxelDensityAblation.h" // development-only world-changing measurements
+#include "VoxelTilePostReach.h" // worker-local landing/structural post reach bits
 
 //=============================================================================
 // SEAL DE FRONTIÈRE / BOUNDARY SEAL
@@ -193,6 +194,17 @@ FORCEINLINE void VF_ApplyOriginLandingAirMC(float& Density,
     float WorldX, float WorldY, float WorldZ,
     float StrateTopZ, float StrateBottomZ, float SealThickness, float BaseDensity, float Radius)
 {
+    if (!VoxelGenLOD::IsOriginLandingReachable()
+        && !VoxelGenLOD::IsTilePostReachBypassActive())
+    {
+        VoxelGenLOD::CompareSkippedPost(Density, [&]()
+        {
+            VF_ApplyOriginLandingAirMC(
+                Density, WorldX, WorldY, WorldZ,
+                StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);
+        }, VoxelGenLOD::ETilePostComparisonKind::OriginAir);
+        return;
+    }
     float InternalDensity = -Density;
     VF_ApplyOriginLandingAir(InternalDensity, WorldX, WorldY, WorldZ,
         StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);
@@ -204,6 +216,17 @@ FORCEINLINE void VF_ApplyOriginLandingFloorMC(float& Density,
     float WorldX, float WorldY, float WorldZ,
     float StrateTopZ, float StrateBottomZ, float SealThickness, float BaseDensity, float Radius)
 {
+    if (!VoxelGenLOD::IsOriginLandingReachable()
+        && !VoxelGenLOD::IsTilePostReachBypassActive())
+    {
+        VoxelGenLOD::CompareSkippedPost(Density, [&]()
+        {
+            VF_ApplyOriginLandingFloorMC(
+                Density, WorldX, WorldY, WorldZ,
+                StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);
+        }, VoxelGenLOD::ETilePostComparisonKind::OriginFloor);
+        return;
+    }
     float InternalDensity = -Density;
     VF_ApplyOriginLandingFloor(InternalDensity, WorldX, WorldY, WorldZ,
         StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);