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/CheckCrossingHashes.py

@@ -0,0 +1,25 @@
+#!/usr/bin/env python3
+"""Compare all L18 crossing configurations to the L14 24 m crossing hashes."""
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[4]
+RUNS = ROOT / "Saved/Linux/l18-crossing"
+PATTERN = re.compile(
+    r"^\[Pipeline(?:FullDetailHash|WholeViewHash|LODHash|ExactQueryHash)\].*$", re.M)
+failed = []
+logs = sorted(RUNS.glob("*-w*-r*.log"))
+for path in logs:
+    parts = path.stem.split("-")
+    compiler = parts[0]
+    reference_compiler = "clang" if compiler == "clang" else "gcc"
+    reference_path = ROOT / f"Saved/Linux/l14-final-crossing-24m-{reference_compiler}-w12.log"
+    expected = PATTERN.findall(reference_path.read_text(encoding="utf-8"))
+    actual = PATTERN.findall(path.read_text(encoding="utf-8", errors="replace"))
+    if actual != expected:
+        failed.append(path.name)
+        print(f"[CrossingReferenceMismatch] {path.name} expected_rows={len(expected)} "
+              f"actual_rows={len(actual)}")
+print(f"[CrossingSurfaceHashGate] reference=L14-final-crossing-24m-W12 rows={len(logs)} "
+      f"result={'FAIL' if failed else 'PASS'}")
+raise SystemExit(1 if failed or len(logs) != 14 else 0)