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

CODEMAP.md

@@ -208,14 +208,16 @@ all eight archetypes; there is no other path. See [Docs/archive/OPSTACK-PLAN.md]
 | `FVoxelBoxHypotheses` + `FVoxelBoxSdfInterval` + `VF_ForceHypotheses` / `VF_FoldEffect` / `VF_FoldOp` | The fold that turns a stack into an `EVoxelTileClass`, carrying the SDF interval between independent writers and consumers. Unknown is biased toward `Mixed`; the mapping and soundness rule are written out in the header. |
 
 ### 3.2c Structural primitives — `Public/VoxelDensityPrimitives.h`
-`VF_ApplyOriginSpine` · `VF_ApplyBoundarySeal` · `VF_ApplyPassageCarving` · `VF_ApplyXYEdgeSeal` —
+`VF_ApplyOriginSpine` · `VF_ApplyBoundarySeal` · `VF_ApplyUpperBoundarySeal` · `VF_ApplyPassageCarving` · `VF_ApplyXYEdgeSeal` —
 the four world invariants every archetype appends, with landing/floor geometry joined by the
 manager passage post, **moved here 2026-07-27** so the generator and
 the operator stack share ONE copy. The XY helper is a radial smooth ramp in actor-space and is a
 true no-op when `WorldRadiusVoxels == 0`; `VF_XYEdgeSealForcedMarginOverBox` is the shared sound
 proof for `ClassifyBox` and T1.d's global shell skip. `VoxelGenerator.cpp` keeps same-named
 `static FORCEINLINE` forwarders for `ApplyBoundarySeal` and `ApplyOriginSpine`; the passage and
-edge helpers are called directly. **Convention: INTERNAL (positive = solid); `VF_ApplyXYEdgeSealMC` is the output wrapper.**
+edge helpers are called directly. `VF_ApplyUpperBoundarySealValue` contains the common upper-band
+math used by both one-sided and two-sided seals. **Convention: INTERNAL (positive = solid);
+`VF_ApplyXYEdgeSealMC` is the output wrapper.**
 
 ### 3.2d Operator stack — `Public/VoxelDensityOpStack.h` + `Private/VoxelDensityOpStack.cpp`
 ⚠️ **This IS the game's generation path.** `GetDensityAt` builds the stack in its per-chunk refetch
@@ -1137,6 +1139,17 @@ Implementation and evidence: [`Standalone/Research/WorldAssemblerM3/Linux9/REPOR
 The base manifest field is authoritative for tile generation and exact queries. See ARCHITECTURE
 §8.36 and [`Standalone/Research/WorldAssemblerM3/Linux13/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux13/REPORT.md).
 
+### 3.23 Shared strate boundaries and FloatingIslands seal — World Assembler M3 Linux18
+
+| Symbol | Canonical file | Role |
+|---|---|---|
+| `FStandaloneShapeWorld::FResolvedStratumBounds`, `MakeResolvedStratumBounds`, `GetResolvedStratumBounds` | `Standalone/Core/src/ShapeWorldInternal.h` | Resolves each strate's physical lower/upper range and seal width for layer ranges, far-field sample protection, and far-shell continuity. Native FloatingIslands uses its plan's owner/Core ratio; SurfaceWorld retains its `32` Core voxels per slot chunk. |
+| `VF_ApplyUpperBoundarySeal` / `VF_ApplyUpperBoundarySealValue` | `Standalone/Source/VoxelForge/Public/VoxelDensityPrimitives.h` and `Source/VoxelForge/Public/VoxelDensityPrimitives.h` | Shared one-sided top seal used by version-3 native FloatingIslands; the two-sided seal reuses the same upper-band math. |
+
+Geometry version 3 removes the FI lower all-XY seal while retaining its ceiling through the shared
+upper-bound path. Versions 1 and 2 retain their serialized two-sided behavior. See ARCHITECTURE §8.37
+and [`Standalone/Research/WorldAssemblerM3/Linux18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux18/REPORT.md).
+
 ## 4. The density pipeline (most-edited hot path)
 
 ### 4.1 `GetDensityWithParams` (TunnelNetwork) — VoxelGenerator.cpp:3749