docs(assembler): round L20 -- design for director / generators / mesher, layout in metres

Standalone/DESIGN_STRATA_UNITS_AND_LAYERS.md: maps today's code to the three layers, ranks 9
violations (top: no single physical stratum-bounds/interface record; SurfaceWorld 8 m vs
FloatingIslands 4 m per chunk), defines the director-resolved generator request/result contract,
moves seals/caps/voids/interfaces into shared composition, and proposes an explicit LOD-stable
interface surface for the L19 ledges. 6-step migration plan with "worlds don't move" checks;
owner decisions listed. No code or world change.

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

Standalone/Research/WorldAssemblerM3/Linux20/REPORT.md

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+# Linux20 — design research for metre-based strata and generation layers
+
+**Date:** 2026-10-20  
+**Scope:** read current owner/Core code and L19 evidence; measure unit conversions and LOD ring
+errors; write the L20 design only.
+
+## Findings
+
+- `CHUNK_SIZE` is 32 and legacy `VOXEL_SIZE` is 12.5 cm, so the owner chunk is 4 m
+  (`Source/VoxelForge/Public/VoxelTypes.h`). Core defaults to 25 cm and keeps 32 voxels per tile.
+- `BuildSurfaceStratumPlan` computes SurfaceWorld layout from `Slot.*ChunkZ × CHUNK_SIZE` in Core
+  voxel units. This is 32 × 25 cm = **8 m per slot chunk**. L18 records this as the retained
+  SurfaceWorld `topChunkZ × 32` interpretation.
+- `BuildFloatingIslandStratumPlan` serializes owner/Core voxel size and the reduced ratio. At the
+  default 12.5/25 cm ratio, its slot chunk is 32 × 0.5 = 16 Core voxels = **4 m**. Core reverses
+  sample Z using `GetCoreVoxelsPerOwnerChunk` in `ShapeWorldInternal.h`.
+- This factor-of-two difference is real, but it is not enough to claim that it caused the L19 ring
+  heights. L19's ring evidence points to LOD-dependent extraction of the lower SurfaceWorld field.
+  It does mean there is no single metres-per-chunk conversion that both preserves the current native
+  bounds and makes their shared slot indices one physical interface. The design records the current
+  resolved bounds and treats a later interface unification as an explicit change.
+- The L19 FloatingIslands-camera table reports LOD1→2 median / p95 / max = 147.141 / 148.403 /
+  149.862 cm, zero no-hit samples; LOD2→3 = 128.603 / 200.789 / 773.318 cm, with 8 no-hit samples
+  among 308. LOD1/2/3 step sizes at 25 cm are 0.5/1/2 m. The ring audit excludes skirts and masked
+  triangles. Native far-ground refinement starts at LOD4, so it cannot fix those two transitions.
+- The exact location and current geometry evidence are from `Linux19/REPORT.md`: lower SurfaceWorld
+  terrain at approximately −60…−73 m; its terrain triangles remain visible below FloatingIslands.
+  The worst sampled LOD1→2 roots are −60.497 m and −61.996 m. The LOD2→3 pair is −73.080 m and
+  −65.347 m.
+
+## Layer map
+
+The owner-facing layout is composed by `VoxelStrateComposer.cpp`, stored/loaded by
+`VoxelSeasonManifest.cpp`, and resolved into `FStrateSlot`s by `VoxelStrateManager.cpp`. The Core
+snapshot adapter (`VoxelForgeCoreAdapter.cpp`) builds separate SurfaceWorld and FloatingIslands
+plans. Core streams work from `VoxelForgeCore.cpp` and evaluates native fields in
+`WorldAssemblerInternal.h` / `FloatingIslandsInternal.h`; legacy `UVoxelGenerator::GetDensityAt`
+and operator-stack/cave paths remain in `VoxelGenerator.cpp`, `VoxelDensityOpStack.cpp`, and
+`VoxelCaveMorphology.cpp`.
+
+Shared pieces already include Core's FP-contract preamble, fixed-point field code, feature bounds,
+column caches, and interval proofs. `ShapeWorldInternal.h` still chooses among native fields, manager
+state, and the copied legacy evaluator. `World::generateTile` in `VoxelForgeCore.cpp` owns MC setup,
+recipe feature branches, native SurfaceWorld far-ground/cap refinement, and recipe identity salts.
+`FarShellsInternal.h` adds shared boundary ceilings and GPU far-shell repairs; its connector repair
+input is prepared from FloatingIslands geometry in `ShapeWorldInternal.h`.
+
+The complete mapping, boundary proposal, ranked violations, generator contract, and migration plan
+are in [`DESIGN_STRATA_UNITS_AND_LAYERS.md`](../../../DESIGN_STRATA_UNITS_AND_LAYERS.md).
+
+## Direction review
+
+The proposed metre authoring, 25 cm per-strate default, fixed 32³ internal tiles, and separation of
+director/generators/shared composition/mesher are sound. Two qualifications are recorded in the
+design:
+
+1. “Units cost nothing” is true for metre conversion itself. Voxel size affects the physical width
+   of 32³ tiles and the number of tiles/tasks/caches; halving spacing in XYZ implies about 8× the
+   density samples per physical volume, with more per-tile overhead on top.
+2. A no-movement migration cannot use one global chunk-to-metre multiplier. Current Core paths use
+   8 m per SurfaceWorld slot chunk and 4 m per FloatingIslands slot chunk at defaults. Preserve and
+   report those current resolved bounds first; decide separately whether to leave any gap/overlap or
+   intentionally make the new metre-authored interface contiguous.
+
+The L19 density/mesh evidence supports LOD extraction as the direct cause of the measured lower-ground
+ring steps. A shared explicit ceiling surface is the right approach for seals/caps, but not sufficient
+for those upward-facing ground ledges. Step 5 therefore exposes the SurfaceWorld ground profile to a
+generic explicit-surface/transition contract and remains subject to the protected LOD2 approval.
+
+## Output and worktree status
+
+No source, manifest, generator, mesher, or build input was changed. No build or world run was made.
+Thus no new W12 hash claim is needed for this documentation-only round. The L19 reference hashes
+remain the expected equality gates for later source changes:
+
+- SurfaceWorld whole view: `c10697a8f48380c7 / 4197fc1072f0ff58 / 7b3e616ec9459851 / c879e35592bf33e5`
+- FloatingIslands whole view: `00bc3607a7839854 / 4baa66127c2b0892 / e11ef05e851a1517 / 1278bc4b1fffb869`
+
+The pre-existing untracked `Standalone/Research/WorldAssemblerM3/Linux18/exports/` directory was
+left untouched. No commit, push, or stash was made.
+