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

ARCHITECTURE.md

@@ -73,7 +73,7 @@ is the only density path; a degenerate (zero-height) strate has none and is open
 | Maze | `FMazeGenerationParams` | `BuildMazeStack` | origin-directed spanning-tree corridors on a 3D lattice, with capped loop edges and a thread-local per-cell capsule cache |
 | SurfaceWorld | `FSurfaceGenerationParams` | `BuildSurfaceStack` | heightfield terrain: domain-warped continents+ridged mtns+detail, a low-freq **relief map** (`M`) that scales mountains/elevation for plains↔highland variety, **F20 heightfield terrain ops** (`Surface|Ops` — all default off ⇒ byte-identical): **Cliff** (slope-gated steepening — where the analytic structural slope > `CliffSlopeThreshold`, push the height away from the local mean by `CliffSharpness` ⇒ gentle slopes become sheer walls/canyon faces that hug real steep ground, gentle areas untouched; 4 structural resamples only when enabled), **Terrace** (relief-gated plateau quantize + `TerraceHardness` soft-round↔crisp-mesa), **LayerLines** (sedimentary sine shelves, slope-expressed) — pure per-column height remaps in the single oracle `ComputeSurfaceTerrainZ` (`SampleSurfaceStructuralZ` = pre-op raw height, re-sampled at an XY offset for Cliff's slope), biome-selected + border-blended for free via each biome's `SurfaceParams` + the height output-lerp; plus **phase-2 Overhang** (the first VOLUMETRIC op — real jutting shelves like a cliff lip): in `FOverhangShelfMod`, for AIR voxels in the window `(TerrainZ, TerrainZ+OverhangHeight]` above a steep slope, the heightfield is re-sampled UPHILL (toward the cliff) by a reach that GROWS with height and unioned in. Low in the window the shift is ~0 (borrows nearby low rock ⇒ stays air over the void); high up it reaches the far cliff (solid) ⇒ a shelf attached to the cliff, tapering out over the void with air underneath. Per-column `OverhangAmp`(=strength·slope-gate) + unit uphill dir `(DirX,DirY)` are resolved once in `ComputeSurfaceColumn` — the gradient sampled at the REACH scale (`OverhangReach`) so a point out over the void can "see" the cliff to know which way is uphill — and cached on `FSurfaceColumn`. It's genuine 3D (per-voxel structural re-eval), so it's gated hard to steep overhang columns; the union only ADDS rock (never removes), capped at `TerrainZ+OverhangHeight`, so `ClassifyTile` forces Mixed only in `(TerrainZ, TerrainZ+OverhangMargin]` (upward-only; margin = max `OverhangHeight`) — a shelf never holes a trivially-skipped tile, and only the thin cliff-edge band of air tiles is woken (NOT far-field like phase-3 spikes/holes). Overhang undersides classify as ground rock by the F17 rule (down-facing but below `TerrainZ`). Known v1 limit: applies at all LODs (Step-agnostic) — may alias far; gate to fine tiles later. Beaches at water line, high sky-cap ceiling. The cap is shapeable terrain in its own right (`ComputeSurfaceCeiling`, `Surface|Sky` params: `CeilingUndulation` broad inverted hills/valleys, `CeilingRidgeStrength` hanging ridgelines, `CeilingRoughness`+freq fine bumps, `CeilingWarp*`) — defaults (strengths 0, freq 0.04) = old flat-ish cap. (`Surface|Macro` params = the cheap precursor to biomes; `ReliefStrength=0` ⇒ old uniform terrain.) **Sky-cap vs ground is a PER-TRIANGLE surface class (F17), not a per-tile verdict**: the mesher classifies each unique vertex on the **worker** — only down-facing verts (`N.Z<-0.1`) pay a memoized `GetSurfaceHeightAt` column query; nearer `CeilSurf` ⇒ sky-cap, nearer `TerrainZ` ⇒ terrain overhang stays ground (a future SurfaceWorld cave roof — down-facing but below `TerrainZ` — also lands ground by the same rule; non-surface strates always ground). Triangles take the majority class of their 3 verts and are packed as **two contiguous index runs** (ground then cap, `FVoxelMeshData::NumCeilingTriangles`; skirts inherit their source triangle's run) → RMC **polygroups 0/1** → one **section per non-empty group** (`ApplyMeshToTile`, material slot = group index): slot 0 = strate `OverrideMaterial`/default (min-corner chunk), slot 1 = `CeilingMaterial` resolved at the tile's **TOP chunk** (mid as gap fallback, then min; fallback: ground material) — a coarse tile is 2^level chunks tall, so its min corner can sit in a lower strate/gap while the cap belongs to the strate above (this was the "far cap = ground material" residue), so the shadowless overhead rock is tinted separately instead of reading flat/bright. Shadow is **per section** via `FRealtimeMeshSectionConfig::bCastsShadow` (NOT the component `SetCastShadow` — RMC's proxy ignores the component flag; this is also why level≥2 far tiles only stopped casting once the section flag was wired): ground casts at level≤1, the cap section never casts, so the rock ceiling never shadows the terrain below it. History: v1 was a game-thread centre height-oracle (misclassified coarse far tiles → terrain material on the cap underside); v2 a whole-tile worker normal VOTE — which painted **mixed coarse tiles** (one far tile spanning terrain AND cap) entirely with the winner's material, and put sky material under terrain overhangs. The per-triangle class fixes both and is the identity channel caves/F8 will reuse. |
 | VerticalShafts | `FVerticalShaftParams` | `BuildVerticalShaftStack` | full-height shafts + horizontal connectors + partial ledges |
-| FloatingIslands | `FFloatingIslandParams` | `BuildFloatingIslandStack` | asymmetric islands: flat land top + underside tapering to a point, lobed (domain-warped) outline, in an open void |
+| FloatingIslands | `FFloatingIslandParams` | `BuildFloatingIslandStack` | asymmetric islands: flat land top + underside tapering to a point, lobed (domain-warped) outline, beneath the shared ceiling at its configured strate upper boundary |
 | Underwater | `FStrateGenerationParams` + water | (reuses `BuildTunnelNetworkStack`) | tunnel rock + high water table |
 
 All stacks share the convention: internal **positive=solid**, and `AppendStructuralPost` ends every
@@ -1431,8 +1431,9 @@ density roots, including a 100 cm datum-offset negative control. See
   evaluators keep their recorded cap behavior. The clip joins the exact near density root at the owning
   boundary without changing LOD0–3 density hashes. Future families with planar roofs must declare their
   cap height and bounds through the family plan/provider contract; the shared tile generator consumes
-  that declaration. FloatingIslands declares no planar roof, so open sky above a surface island is the
-  expected view.
+  that declaration. FloatingIslands uses the same resolved strate-boundary data flow: its upper seal
+  follows its plan's chunk range and the shared boundary-seal primitive, while SurfaceWorld keeps its
+  existing heightfield cap.
 
 For each requested far SurfaceWorld XY column at LOD4–8, desired-tile selection adds cap-height
   bands only when they do not overlap an existing or already accepted 3D clipmap tile. The original
@@ -1471,8 +1472,9 @@ neighborhood have installed collision (or are explicitly collision-not-required)
 now** — the active view has ground, horizon and the family-appropriate ceiling, with no exposed
 coverage hole or unexpected sky; (3) **refined soon** — each target LOD resolves within its stated
 time or travel distance after entering range. While moving, report the largest frame/time gap in
-enclosure and maximum distance that unresolved work trails the pawn. FloatingIslands has open sky by
-design, so its enclosure check excludes a planar roof.
+enclosure and maximum distance that unresolved work trails the pawn. FloatingIslands is enclosed by a
+planar roof at the upper boundary of its configured strate range; a taller strate places that roof
+farther above its islands.
 
 The shared `AVoxelWorld::UpdateChunksAroundPosition` scheduler serves both the Core snapshot path and
 Core-off legacy path. It admits the collision-critical 3×3 ring first. The measured S5 order remains
@@ -1840,6 +1842,20 @@ fill and check page tiles against the evaluator they actually sample. This is th
 the Unreal adapter and its asynchronous MPSC/Epoch flow are unchanged. See
 [`Standalone/Research/WorldAssemblerM3/Linux13/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux13/REPORT.md).
 
+### 8.37 Shared strate boundaries and FloatingIslands lower seal — World Assembler L18
+
+`FStandaloneShapeWorld::GetResolvedStratumBounds` supplies one physical lower/upper-bound descriptor
+to layer ranges, far-field sample protection, and far-shell continuity for every strate. SurfaceWorld
+retains its exact `topChunkZ * 32` Core-voxel mapping and native heightfield cap. Native FloatingIslands
+takes its resolved bounds from its plan, whose chunk range is converted through the serialized
+owner/Core voxel ratio. Its version-3 field removes only the lower all-XY seal and calls the same
+`VF_ApplyUpperBoundarySeal` primitive used by the shared two-sided `VF_ApplyBoundarySeal` for its upper
+boundary. The ceiling therefore sits at that strate's own upper boundary, with the common
+`boundarySealSubvoxels` thickness; there is no FloatingIslands-specific height. The recipe range is
+un-inset to match the Unreal adapter. Versions 1 and 2 retain their serialized two-sided seals.
+SurfaceWorld's field and output hashes remain unchanged. See
+[`Standalone/Research/WorldAssemblerM3/Linux18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux18/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