wip(assembler): round S10 -- LOD8 horizon sampled at MC spacing: LOD8 worker time 18.8 s -> 4.3 s

Planar far horizon samples the LOD8 interior at the MC cell spacing, keeping
128-voxel boundary strips; far recipe v3. LOD8 p95 59.8 -> 13.9 ms, spawn
worker-seconds ~41 -> 26.9, full set 7.8 -> 7.1 s, ring 0.49 s, LOD0 1.43 s,
40 m/s ring 75/75. S3/S4 2 cm agreement, 100% cap coverage, W1/W8/W12 and
W1/W8/W16 determinism pass. Report: Standalone/Research/WorldAssemblerM3/Round14.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

ARCHITECTURE.md

@@ -1648,6 +1648,37 @@ FloatingIslands checks passed; the S3/S4 ground, cap and coverage audit passed.
 negative control, hashes, stage ledger and the deferred LOD8 optimization plan are in
 [`Standalone/Research/WorldAssemblerM3/Round13/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round13/REPORT.md).
 
+### 8.31 LOD8 planar far-horizon sampling — World Assembler S10 / M3 Round 14
+
+The planar far horizon is already built from a `Heightfield2D` column profile, but LOD8 previously
+refined ground and cap at a 128-voxel grid even though a 25 cm LOD8 marching-cubes cell spans 512
+voxels (128 m). `BuildNativeHorizonGridCoordinates` now keeps the globally aligned 128-voxel edge
+strips and uses the marching-cubes step through the LOD8 interior. The boundary strips preserve
+shared edge samples; the interior does not build detail smaller than the LOD8 cell spacing. LOD4–7
+retain their prior sample coordinates. No agreement tolerance was relaxed.
+
+This applies to native horizon plans that declare a planar heightfield. Ground and cap still run in
+separate refinement passes, using the same deterministic coordinate lattice. `Heightfield2D` keeps
+its thread-local column cache: S10 records remaining duplicate evaluations across LOD8 tiles and
+workers but does not share cache pages between workers. The `--far-horizon-lod8-step=128` Standalone
+override restores the old sampling density for an A/B control; it does not restore the old v2 hash
+salt. The default sampling change bumps the far recipe hash version from 2 to 3. LOD0–3 hashes and
+density are unchanged.
+
+`VoxelForgeSharedFeatureCost` now reports column evaluation, marching-cubes, ground-refine,
+cap-refine and output-assembly timings separately. Column evaluation is subtracted from each
+overlapping MC/refinement wall interval; output assembly includes the final payload flattening and
+optional diagnostic hashing. Percentiles are computed independently and are not additive. The
+optional LOD8 XY capture uses worker-local vectors and records only diagnostics; it does not affect
+mesh generation or recipe identity.
+
+On the staged 25 cm SurfaceWorld plan, the warmed LOD8 full-tile p95 fell from 59.848 ms to
+13.933 ms. Column evaluation fell from 31.319 ms to 7.478 ms; ground and cap refinement fell from
+11.666/10.029 ms to 1.690/1.330 ms. The S3 ground agreement, S4 cap agreement and 100% cap-ray
+coverage gates still pass at their existing tolerances. BuildHost actor startup, the W1/W8/W16
+matrix, W1/W8/W12 Standalone hashes, the 40 m/s support-ring trace and the S10/S9 captures are
+recorded in [`Standalone/Research/WorldAssemblerM3/Round14/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round14/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