wip(assembler): round L33 -- SurfaceWorld can be an upper strate; FloorProfileBlend measures seams

Route planning for an upper SurfaceWorld failed on long candidate chords over steep relief; chords
are now capped to the arrival/departure radii on an 8 m centre grid with an origin-proximity cost
(21.5 m walk, about 4 deg). The departure passage starts at the SW ground and tunnels down through rock.
FI -> SW -> TN fixture: all 4 styles pass every passage audit on both compilers. FloorProfileBlend
now checks endpoint height/tangent seams (step and tangent-step negative controls detected). Owner
world hashes unchanged; 30/30 determinism; cost within noise.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CSFYgsdW8ejZSyiMf4qRSX

Standalone/DESIGN_WORLD_ASSEMBLER.md

@@ -83,6 +83,12 @@ Costs below are design estimates, not timings. Low means bounded analytic featur
 
 A passage planner must produce a traversable route or pair a non-walkable visual passage with another guaranteed route. A narrow crack or drop cannot be assumed walkable because its centerline is air.
 
+For a SurfaceWorld strate, the supported arrival-to-departure route follows the sampled terrain
+ground and stays within the mouth placement bounds. The inter-strate departure passage owns the
+descent from that ground mouth through rock and bedrock. Treating the bottom of a tall SurfaceWorld
+strate as the route's departure height would ask the walk planner to cross open terrain air; it is
+the wrong endpoint for the intra-strate route.
+
 | Passage type | Planner output | Bound and likely cost |
 |---|---|---|
 | SlopedTunnel | Straight-style path with switchbacks as needed, capsule clearance, supported floor, maximum grade and landing geometry. | Swept capsule bounds plus floor envelope; low–medium. |