wip(assembler): round L18 -- FloatingIslands false floors removed; shared strate ceiling

FI geometry v3 drops the all-XY lower boundary seal that created false floors (3.5/8 m); the
ceiling comes from the shared VF_ApplyUpperBoundarySeal and AppendStratumBoundaryCeilings path
for every generator, at each strate's own configured upper boundary (owner: every strate has a
ceiling). Far-LOD ceiling continuity: 0 gaps at LOD4-8. SurfaceWorld bit-identical to L14
(14 configs + 24 m crossing); FI deterministic (28-row matrix); route parity and proofs pass.
Unreal: FI geometryVersion 2->3 and the shared upper-seal helper extracted (UBT compile needed on
the owner PC). Open: small ceiling holes; lower-stratum LOD ring steps seen through the FI void.

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

Standalone/Core/API.md

@@ -153,8 +153,13 @@ radius range, top flattening, taper, bounded contour warp, roughness, blend, con
 route constraints. `PlanFloatingIslandCluster` emits stable island and connector IDs, a connected
 cluster graph, conservative influence AABBs, and a centerline route. At least the primary chain is
 always planned; `islandDensityQ16` controls up to two optional side sites. Connectors carry a sharp-tag
-ready bit for a later hybrid-mesher adapter. The schema accepts geometry version 1 and 2. Manifests
-without a geometry version retain version-1 evaluation and canonical hashes.
+ready bit for a later hybrid-mesher adapter. The schema accepts geometry versions 1 through 3.
+Version 3 keeps the version-2 island form, removes the all-XY lower seal, and obtains its ceiling from
+the shared upper-strate boundary seal. The resolved upper bound comes from the strate's own chunk
+range; `boundarySealSubvoxels` sets the shared seal thickness and remains a planning margin for island
+features. It does not define a FloatingIslands-specific ceiling height. Versions 1 and 2 retain their
+recorded two-sided boundary seals. Manifests without a geometry version retain version-1 evaluation
+and canonical hashes.
 
 `FloatingIslandFieldV1::densitySubvoxels` evaluates the canonical island and connector field.
 `interval` returns unknown for boxes overlapping any influence AABB and a certified all-air range