feat(passages): §6.5 landings -- you arrive somewhere you can stand

The last unbuilt Tier 1 item. A passage was a bore through rock with no
floor inside the walkable angle, so its mouth was not a standing position
and neither was anything near it -- the departure mouth measured a FOUR-CELL
player-fit component. You arrived and there was nowhere to stand.

Both ends of all 7 passages now open into a landing, sized from the body:
  capsule diameter 2.72 vox      floor >= 12 vox (3 m) to stand and turn
  capsule height 7.04 vox        room >= 12 vox tall, stock mouth gives 18 (4.5 m)
  3-voxel floor slab, tube tangent to it -- no lip to climb out of
  connector 5-vox radius: 2.5 m clear width, 3 m clear height
  ramps over 44 deg get a deterministic level dog-leg

⭐ THE MOUTHS ARE NO LONGER SPECKS:
  archetype        components      largest      arrival-departure
  Maze             309 -> 340      176 -> 3210    43-81  -> 3210-3210
  TunnelNetwork    467 -> 227     1292 -> 3841     2-1   -> 3841-269
  Underwater        86 -> 117       35 -> 3838     3-11  -> 3838-267
  FlatPlain         45 ->  11      259 -> 2130     1-1   ->  268-273
  CrystalChamber   197 ->  90       24 ->  286     1-1   ->  286-274
  FloatingIslands    2 ->   1        3 ->   72     1-1   ->   72-72

Law 1/8 -> 2/7 connected (1 refused on MaxCells), and critically ZERO
connections now run through a four-cell pocket.

Every landing floor was verified against the FINAL density, after all posts:
14/14 pass the 44 deg slope bracket, worst room gradient .059, worst
connector .522. 14/14 deterministic root joins. 0 seal failures.
0 box-verdict violations across 82 proved + 174 mixed, 109,142 voxels.
Op-stack bit-identical, order-independence green over 10 slots / 31
passages / 3 pool orders. Cache 0.665 us rebuild, 0.646 us hot.

Post order: origin spine -> vertical seal -> passage tube + landing floor
-> XY edge seal. Landings clamp 4 voxels inside the vertical seal; the XY
seal stays last and wins at world edges.

⚠️ ARCHITECTURAL NOTE, not a detail: landings join a deterministic annular
ROOT HUB at (0,0), built identically for every endpoint -- no nearest-
component search, no passage ordering. That is what makes the join
guaranteed rather than likely, and it extends the (0,0) spine concept from
a landmark into a structural network. It also means routes tend to pass
through the centre, which is a gameplay consequence worth Jahni's eye.

⚠️ AND IT DOES NOT CLOSE THE LAW. Codex is explicit: the landings guarantee
membership in the new root network, NOT in the archetype's pre-existing fit
component. The whole-strate Maze check still shows a 71.9-voxel gap between
1,235 and 2,938-cell components. So the native corridor networks remain
disconnected from the root network in places. Real, separate, next.

52 build-and-measure cycles, ~9 hours, finding real geometry bugs on the
way (a landing floor overlapping the ramp cells and hiding a floor
discontinuity; the disturbance pass refilling connector headroom).

Build clean, 32/32 success, 0 failed. Showcase regenerated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K8V9my1PL68pmQb2gutZZV

Source/VoxelForge/Public/VoxelDensityPrimitives.h

@@ -87,6 +87,28 @@ FORCEINLINE void VF_ApplyPassageCarving(float& Density, float ModSDF,
     Density = FMath::Min(Density, FMath::Lerp(Density, AirTarget, CarveFactor));
 }
 
+// Landing air is also reasserted after the MC-space disturbance post.  Unlike the legacy tube
+// carve above, this threshold is independent of the incoming density: applying it a second time
+// is therefore bit-stable.  The structural passage op and the post-disturbance backstop can share
+// the same operation without turning an otherwise unchanged live-vs-direct op-stack comparison
+// into a second, deeper smooth carve.  The tube intentionally retains its old current-density
+// interpolation because that is part of the legacy structural path's contract.
+FORCEINLINE void VF_ApplyPassageLandingCarving(float& Density, float LandingSDF,
+    float BaseDensity, float SealThickness)
+{
+    constexpr float PASSAGE_BLEND_RADIUS = 4.0f;
+    if (LandingSDF >= PASSAGE_BLEND_RADIUS) return;
+
+    float CarveFactor = FMath::Clamp(
+        (PASSAGE_BLEND_RADIUS - LandingSDF) / (PASSAGE_BLEND_RADIUS * 2.0f),
+        0.0f, 1.0f);
+    CarveFactor = SmoothStep01(CarveFactor);
+
+    const float AirTarget = -(BaseDensity * 2.0f + SealThickness + 4.0f);
+    const float LandingThreshold = FMath::Lerp(BaseDensity, AirTarget, CarveFactor);
+    Density = FMath::Min(Density, LandingThreshold);
+}
+
 //=============================================================================
 // SPINE DE DESCENTE (0,0) / (0,0) DESCENT SPINE
 //=============================================================================