fix(passages): D-floors now exist inside the inter-strate gap; two stale tests corrected

Investigation of the two tests that had been failing for days (Codex; the fix reviewed and its
field claims re-run by me).

Real bug, introduced by adb34df: gap chunks take the compact bedrock path in GetDensityAt, which
applied the passage modifier but never the build-authored native D-floor. So the walkable passage
floor vanished exactly at the gap/strate seam, in the passages the player uses to cross strates.
Now gap chunks set Result = -Density, then ApplyPassageNativeFloorMC, then VF_ApplyXYEdgeSealMC.
The seal runs on the MC value, in the same order as before.

Stale tests, not code bugs:
- PassageLandsInOpenSpace: since d97373c production passes the cached world/warp seed to
  VF_SuggestLandingPoint, while the test still queried with the default seed. Both queries are
  fixed; the test now allows only exact graph-owned support-floor samples (the production
  bProtectAuthoredTunnelFloor contract), and anything else still fails. 14/14 landing ends,
  5,774/5,774 floor and capsule samples.
- ClassifyTileSoundness: since d38d4ca ClassifyTile covers the cell vertices g=0..CPA, but the test
  still sampled the mesher's outer normal halo. Both "holes" were halo-only (g = CPA+1). The test now
  asserts the current contract. TryProveSealedSolidTile already uses the exact core lattice, so it
  is not affected.

Verified: crossing test PASS (max gate 0.129 s); sealed-proof test 22/22; both classifier variants
pass (600 tiles each). Re-run by the orchestrator against this build: export OBJ SHA-256
b3e5f4c3..9b377 unchanged; capability 20,830 / 10,909 / walk-only 10,909.
Ledger: one DDC startup fatal (crash folder 43101DC8, 04:38) and a set of expected diagnostic
failures while locating the gap-branch omission; one malformed ablation argument (not a valid
ablation).

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

Source/VoxelForge/Private/VoxelGenerator.cpp

@@ -2760,8 +2760,14 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
         float Density = 8.0f;  // bedrock solidity (positive = solid)
         StrateManager->ApplyPassageModifier(
             Density, WorldX, WorldY, WorldZ, 8.0f, 0.0f);
-        VF_ApplyXYEdgeSeal(Density, WorldX, WorldY, WorldRadiusVoxels, EdgeSealThickness, 8.0f);
+        // Gap chunks use the compact bedrock path above instead of the ordinary strate tail.
+        // Native walkable passages still cross those chunks, so compose their build-authored
+        // D-floor here as well; otherwise the floor disappears exactly at the gap/strate seam.
         Result = -Density;
+        StrateManager->ApplyPassageNativeFloorMC(
+            Result, WorldX, WorldY, WorldZ, 8.0f);
+        VF_ApplyXYEdgeSealMC(
+            Result, WorldX, WorldY, WorldRadiusVoxels, EdgeSealThickness, 8.0f);
     }
     else if (StrateManager)
     {