feat(descent): landing ROOMS and a walkable tunnel -- per Jahni's correction

Jahni corrected the design: "the connection between level is a tiny tunnel
connecting both strate, not vertical, it needs to be walkable" and "'the
spine' shouldn't be carved except optionally from the top first strate,
then it's used to have a landing room".

COMPOSER-NOTES §6.8 had said the spine "provides a clean landing space, not
a continuous open shaft" -- and the implementation carved a vertical
cylinder anyway. A vertical cylinder has no walkable floor except at its
bottom, so a carve guaranteeing open AIR guaranteed nothing STANDABLE.
That one omission is behind five of my dead hypotheses this week.

Now:
  origin spine   a finite rounded ROOM per strate, no vertical bore.
                 radius 14 -> 8 m wide, 7.5 m tall
  descent        a walkable tunnel, single-file by design: 3 m mouth,
                 2.5 m waist, gradient capped at a NAMED 15 deg
  exotic styles  VerticalShaft / SpiralDescent / CascadingDrops /
                 CrackCrevice preserved, no longer the default

⚠️ THE GEOMETRY BITES, and it is worth Jahni's eye: at 15 deg a 32 m drop
needs 119.4 m of horizontal run -- about 59.7 m per switchback leg. That is
a long walk between strates. It is the honest consequence of "walkable",
not a tuning artifact, and the alternative is a steeper number.

⭐ AND THE ROOT CAUSE GOT NAMED A THIRD TIME, by Codex mid-run: "the round
tube was hollow air but had no authored support strip, so 'open' still did
not mean 'standable'." Tunnels now carry a floor band, and the root join
terminates in the finite origin room rather than an annular shaft hub.

Measured, commandlet sweep, 8 archetypes x 8 seeds, 64/64 valid:
  origin column has player-fit cells   17/64 -> 63/64
  origin column reachable               0/64 ->  7/64
  arrival landing probes            315/320 fit AND in the arrival component
  tunnel audit    5,916/5,916 floor and capsule samples pass; zero gradient,
                  floor, capsule, geometry or seal failures
  narrow-gap events 0        box verdicts 14 checked, 0 violations

⚠️ arrival->departure is 6/64. The landings and tunnels are now provably
walkable; what remains is the INTRA-STRATE vertical connectivity problem
found in 6b91669 -- floors at different levels inside a strate with nothing
joining them. Separate and larger. Not tuned toward.

⚠️ PROCESS: this took 5.7 hours, of which Codex used 4 SECONDS of CPU. The
rest was polling the 73-minute automation suite at ~10% machine load. The
commandlet exists to avoid exactly that. New rule recorded: iterate with the
commandlet, full suite ONCE at the end.

Editor + Shipping builds clean. Automation 32/32, 0 failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aheo2crrZmHuYtPpdY2Cb

Source/VoxelForge/Private/VoxelGenerator.cpp

@@ -296,6 +296,14 @@ static FORCEINLINE void ApplyOriginSpine(float& Density, float WorldX, float Wor
         StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);
 }
 
+static FORCEINLINE void ApplyOriginLandingFloor(float& Density,
+    float WorldX, float WorldY, float WorldZ,
+    float StrateTopZ, float StrateBottomZ, float SealThickness, float BaseDensity, float Radius)
+{
+    VF_ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        StrateTopZ, StrateBottomZ, SealThickness, BaseDensity, Radius);
+}
+
 // DISTURBANCE LAYER — the "wow" post-process. Operates on the FINAL MC density
 // (negative = solid, positive = air), AFTER the archetype produced its terrain, so
 // it works uniformly for every generator type. Stays inside the seal bands so it can
@@ -1061,11 +1069,40 @@ float UVoxelGenerator::GetDensityAt(float WorldX, float WorldY, float WorldZ) co
                 break;
             }
         }
+        // Origin rooms are structural too. Reassert their air before either support writer so a
+        // bridge/ridge cannot plug a landing, while the two floor writers remain last.
+        VF_ApplyOriginLandingAirMC(
+            Result, WorldX, WorldY, WorldZ,
+            CP_Dist.StrateTopWorldZ, CP_Dist.StrateBottomWorldZ,
+            CP_Dist.BoundarySealThickness, LandingBaseDensity, OriginSpineRadius);
         StrateManager->ApplyPassageLandingAirMC(
             Result, WorldX, WorldY, WorldZ, LandingBaseDensity,
             CP_Dist.BoundarySealThickness);
+        VF_ApplyOriginLandingFloorMC(
+            Result, WorldX, WorldY, WorldZ,
+            CP_Dist.StrateTopWorldZ, CP_Dist.StrateBottomWorldZ,
+            CP_Dist.BoundarySealThickness, LandingBaseDensity, OriginSpineRadius);
         StrateManager->ApplyPassageLandingFloorMC(
             Result, WorldX, WorldY, WorldZ, LandingBaseDensity);
+        // A walkable tunnel's guaranteed air core is the final structural writer above its
+        // floor. This prevents a nearby passage's floor slab from capping the active route;
+        // the tunnel-floor predicate is false on the active floor itself, so support remains.
+        StrateManager->ApplyPassageTunnelAirMC(
+            Result, WorldX, WorldY, WorldZ, LandingBaseDensity,
+            CP_Dist.BoundarySealThickness);
+        VF_ApplyOriginLandingAirMC(
+            Result, WorldX, WorldY, WorldZ,
+            CP_Dist.StrateTopWorldZ, CP_Dist.StrateBottomWorldZ,
+            CP_Dist.BoundarySealThickness, LandingBaseDensity, OriginSpineRadius);
+        StrateManager->ApplyPassageLandingAirMC(
+            Result, WorldX, WorldY, WorldZ, LandingBaseDensity,
+            CP_Dist.BoundarySealThickness);
+        VF_ApplyOriginLandingFloorMC(
+            Result, WorldX, WorldY, WorldZ,
+            CP_Dist.StrateTopWorldZ, CP_Dist.StrateBottomWorldZ,
+            CP_Dist.BoundarySealThickness, LandingBaseDensity, OriginSpineRadius);
+        StrateManager->ApplyPassageLandingRoomFloorMC(
+            Result, WorldX, WorldY, WorldZ, LandingBaseDensity);
     }
     else
     {
@@ -2225,6 +2262,9 @@ float UVoxelGenerator::GetDensityWithParams(float WorldX, float WorldY, float Wo
             Density, WorldX, WorldY, WorldZ,
             Params.BaseDensity, Params.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        Params.StrateTopWorldZ, Params.StrateBottomWorldZ,
+        Params.BoundarySealThickness, Params.BaseDensity, OriginSpineRadius);
 
     // Fourth structural post: the XY edge wins over a passage near the rim.
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
@@ -2448,6 +2488,9 @@ float UVoxelGenerator::GetSlabDensity(float WorldX, float WorldY, float WorldZ,
             Density, WorldX, WorldY, WorldZ,
             Params.BaseDensity, Params.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        Params.StrateTopWorldZ, Params.StrateBottomWorldZ,
+        Params.BoundarySealThickness, Params.BaseDensity, OriginSpineRadius);
 
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
         WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
@@ -2569,6 +2612,9 @@ float UVoxelGenerator::GetMazeDensity(float WorldX, float WorldY, float WorldZ,
             Density, WorldX, WorldY, WorldZ,
             Params.BaseDensity, Params.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        Params.StrateTopWorldZ, Params.StrateBottomWorldZ,
+        Params.BoundarySealThickness, Params.BaseDensity, OriginSpineRadius);
 
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
         WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
@@ -2833,6 +2879,9 @@ float UVoxelGenerator::SurfaceDensityFromColumn(float WorldX, float WorldY, floa
             Density, WorldX, WorldY, WorldZ,
             S.BaseDensity, S.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        S.StrateTopWorldZ, S.StrateBottomWorldZ,
+        S.BoundarySealThickness, S.BaseDensity, OriginSpineRadius);
 
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
         WorldRadiusVoxels, EdgeSealThickness, S.BaseDensity);
@@ -3223,13 +3272,20 @@ EVoxelTileClass UVoxelGenerator::ClassifyTile(const FIntVector& OriginVoxels, in
     {
         bCanAir = false;
     }
-    if (OriginSpineRadius > 0.0f)
+    if (StrateManager->AnyOriginLandingNearBox(
+            FVector(MinX, MinY, MinZ), FVector(MaxX, MaxY, MaxZ)))
     {
-        // Spine (0,0) : colonne XY rayon R + blend 3 (cf. ApplyOriginSpine). Test cercle/boîte XY.
-        const float Reach = OriginSpineRadius + 3.0f;
-        const float CX = FMath::Clamp(0.0f, (float)MinX, (float)MaxX);
-        const float CY = FMath::Clamp(0.0f, (float)MinY, (float)MaxY);
-        if (CX * CX + CY * CY <= Reach * Reach) bCanSolid = false;
+        // The origin post is a room carve, not a full-height spine. Its conservative manager
+        // guard is per tile, so it retains the hot-path invariant while covering every strate's
+        // room AABB and its four-voxel carve blend.
+        bCanSolid = false;
+    }
+    if (StrateManager->AnyOriginLandingFloorNearBox(
+            FVector(MinX, MinY, MinZ), FVector(MaxX, MaxY, MaxZ)))
+    {
+        // The same origin room also owns a guaranteed support slab, so a tile touching only that
+        // slab cannot be proved uniformly air.
+        bCanAir = false;
     }
 
     // ── Catégorisation par Z du treillis : gap bedrock = solide ; hors layout = air constant ;
@@ -4556,6 +4612,9 @@ float UVoxelGenerator::GetVerticalShaftDensity(float WorldX, float WorldY, float
             Density, WorldX, WorldY, WorldZ,
             Params.BaseDensity, Params.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        Params.StrateTopWorldZ, Params.StrateBottomWorldZ,
+        Params.BoundarySealThickness, Params.BaseDensity, OriginSpineRadius);
 
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
         WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);
@@ -4717,6 +4776,9 @@ float UVoxelGenerator::GetFloatingIslandDensity(float WorldX, float WorldY, floa
             Density, WorldX, WorldY, WorldZ,
             Params.BaseDensity, Params.BoundarySealThickness);
     }
+    ApplyOriginLandingFloor(Density, WorldX, WorldY, WorldZ,
+        Params.StrateTopWorldZ, Params.StrateBottomWorldZ,
+        Params.BoundarySealThickness, Params.BaseDensity, OriginSpineRadius);
 
     VF_ApplyXYEdgeSeal(Density, WorldX, WorldY,
         WorldRadiusVoxels, EdgeSealThickness, Params.BaseDensity);