feat(gen): Jahni's playtest corrections -- rooms flattened, roads deleted

From walking a TunnelNetwork preset at (0,0). Four corrections, and two of
them were the same mistake.

2a. The (0,0) landing room is sized to host a FUTURE straight vertical
    shaft from the surface, with flooring around the origin itself. The
    room is the landing for a shaft the player digs later.
2b. The inter-strate descent no longer starts at the spine -- placed by
    deterministic hash within a radius around (0,0). The spine room is for
    the surface shaft; the descent is elsewhere.
2c. ⛔ DELETED the landing->hub connector roads from 413c15d. Jahni:
    "its straight up making other tunnels straightly through everything...
    they have nothing to do here." Zero references remain.
2d. ⭐ FLATTEN THE ROOM JOINS instead. Where two spherical rooms intersect
    and leave a lip above head height, the join is brought down to a
    walkable level.

⭐ 2c AND 2d ARE ONE CORRECTION. I solved "rooms do not connect" by
TUNNELLING BETWEEN THEM when the answer was MAKING THEIR FLOORS MEET. The
81-run sweep had already said so -- zero narrow-gap events, every failure a
vertical severance, renders showing "a sharp diagonal ledge descending into
black open space". A radial corridor to the origin could never have fixed
that; flattening the joins is aimed at exactly it.

2e. Descent tunnel 1.5x wider, per "1.5x bigger wouldn't be too much".
2f. Corridors WANDER. Jahni, from the renders: "its not a maze, its still a
    latice made straight paths." The spanning tree changed WHICH edges
    exist, not what an edge looks like -- every corridor was a straight
    axis-aligned capsule, every junction 90 degrees. Now hash-jittered
    chains, seam-safe like the tree.

RENDERER: 192x128 -> 512x288, and cameras are now seeded from the
player-fit set -- "a density-checked pose from the exact player-fit walk
mask". Previously they sat INSIDE ROCK looking at back-faces, which Jahni
spotted instantly and no metric could. First genuinely legible pictures of
this world.

ParallelFor landed at 6 sites in VoxelStrateMeasure before I stopped the
run; the rest of that work continues next.

⚠️ AND THE RENDERER IS ARCHITECTURALLY WRONG, per Jahni: "why, in engine,
does it take a minute to have clear view of thousands of angles, but in our
renderer it takes ages?" Because the engine MESHES ONCE and rasterises --
~32k density calls per chunk, then every angle is free. I RAYMARCH: tens of
millions of density calls PER IMAGE, nothing reused between frames. We
already have the mesher and an export mode. Next task renders from the
mesh.

Also from him: runs must stay under 30 minutes. This one hit 33 and
produced 2 images.

Build clean. Committed as a checkpoint -- the task was stopped mid-Part-3,
not finished.

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

Source/VoxelForge/Public/VoxelSettings.h

@@ -393,9 +393,10 @@ public:
 	//=========================================================================
 
     // Authored radius (voxels) of the guaranteed (0,0) landing room in EVERY strate, regardless
-    // of archetype. The §6.5 body-sized formulas turn it into a flat-floored room; this setting
-    // no longer bores a vertical column. 0 = disabled. The room is top-anchored inside each
-    // strate, and the seals remain solid until a progression passage is opened.
+    // of archetype. The §6.5 body-sized formulas turn it into a flat-floored room reserved for a
+    // future straight shaft of diameter 2 * this radius. At the default 14-voxel radius that is
+    // 28 voxels / 7 m, with a three-voxel / 0.75 m floor ledge around it. This setting does not bore the
+    // shaft today: lower seals remain solid until a progression passage is opened.
     //   10-14 → compact landing room (default) · 20+ → broad landing room
 	UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = "Voxel|Spine", meta = (ClampMin = "0.0"))
 	float OriginSpineRadius = 14.0f;