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/Research/WorldAssemblerM3/Linux18/BuildResources.md

@@ -0,0 +1,9 @@
+# L18 build resources
+
+Limits: CPUs 0–11, `-j12`, normal priority, 6 GiB address-space cap. Governor stayed `powersave`.
+These are the final builds after the cell-coverage and owner ceiling corrections.
+
+- Clang 19, `-ffp-contract=off`: exit 0, 31.689 s, peak process RSS 436,088 KiB, peak VMS 586,240 KiB.
+- GCC 14, default FP contraction: exit 0, 48.648 s, peak process RSS 903,168 KiB, peak VMS 1,014,716 KiB.
+- Largest measured runtime process was the final Clang gate run at 897,276 KiB RSS. The determinism matrix peaked at 603,172 KiB and the crossing matrix at 548,664 KiB.
+- An earlier GCC build was interrupted by owner steering before producing a binary; the final GCC build above completed successfully.