wip(assembler): round L19 -- strate ceilings closed at every LOD; ledges diagnosed

The shared boundary-cap path now emits a cap wherever the field is solid just below a strate's
configured top (it previously required a solid-to-air crossing there) and checks a 5x5 perimeter
grid before suppressing a fallback cap. Strict audit: 0 misses / 0 edge gaps, LOD0-8.
farHorizonVersion 3 -> 4. SurfaceWorld LOD0-2, full detail and exact queries unchanged; 28/28
matrix plus 14/14 crossing; route parity and proofs pass.
Ledges: the lower stratum's terrain at the FI/SW interface, seen through the FI void, sampled at
0.5/1/2 m at LOD1-3 (up to 1.5 m / 7.7 m ring steps). Fix deferred to the units/layers round.

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

ARCHITECTURE.md

@@ -1856,6 +1856,19 @@ un-inset to match the Unreal adapter. Versions 1 and 2 retain their serialized t
 SurfaceWorld's field and output hashes remain unchanged. See
 [`Standalone/Research/WorldAssemblerM3/Linux18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux18/REPORT.md).
 
+### 8.38 Closed shared boundary ceilings and LOD sampling audit — World Assembler L19
+
+`AppendStratumBoundaryCeilings` fills a candidate cell when the authoritative field is solid just
+inside that strate's configured upper boundary. It does not require the raw field to cross back to
+air above the boundary: the ceiling itself closes the strate even when the field continues solid
+through that plane. This avoids isolated holes where a coarse lattice node has no zero crossing.
+`FFarMeshColumnIndex::HasDownwardCellCoverage` still suppresses a fallback quad only after its
+interior and perimeter probes all hit the existing downward face. The L19 `farHorizonVersion` is 4
+for this shared boundary-cap change; hashes remain direct at LOD0–3 and salted at LOD4–8. No density
+query, stratum position, scale, or asynchronous route changes. The strict LOD0–8 roof and paired-edge
+audit and the matched before/after captures are in
+[`Standalone/Research/WorldAssemblerM3/Linux19/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux19/REPORT.md).
+
 ## 9. Multiplayer model (listen-server first, dedicated-friendly)
 
 > **Status: transport is DESIGN ONLY — nothing is networked in-tree yet** (no `Replicated`/`HasAuthority`/RPCs; a