wip(assembler): round L9 -- LOD1 skips proven-uniform blocks: 95% fewer samples, view ~2% faster

Step-2 (LOD1) tiles of native SurfaceWorld now prove 8-cell blocks uniform from exact heightfield
column extrema over the padded block (cells + normal halo) and skip their density fill; features,
edits, mixed strata and Unknown verdicts still fill. LOD1 samples 42,875 -> 2,063 per tile.
Audit: 80,502 skipped blocks (3 seeds) + 9,216 island blocks force-filled, 0 violations. All
SurfaceWorld and FloatingIslands hashes unchanged at W1/W8/W12, Clang off and GCC default.
Reviewer interleaved W12, no hashing: whole view 1.439 -> 1.409 s. L8 phase probes removed;
--profile-lod-work and --audit-uniform-block-proofs are opt-in.

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

CODEMAP.md

@@ -1118,6 +1118,16 @@ restores the S13 path for the negative control. Runtime CVar capture lives in
 cold-path decision are in
 [`Standalone/Research/WorldAssemblerM3/Round18/REPORT.md`](Standalone/Research/WorldAssemblerM3/Round18/REPORT.md).
 
+### 3.21 LOD1 uniform-block proof — World Assembler M3 Linux9
+
+`FStandaloneShapeWorld::IsNativeSurfaceBox` gates the step-2 proof to a single native SurfaceWorld
+stratum. `ClassifyBoxDetailed(..., bUseNativeLatticeProof)` delegates to
+`Heightfield2D::classifyLatticeBox`, whose ground/cap extrema cover the MC lattice and normal halo.
+`UVoxelMarchingCubesMesher::GenerateMesh` sparsely fills only Mixed blocks and their sample halo;
+Mixed/Unknown remains on the fill path. The headless `--audit-uniform-block-proofs` forces sample
+verification, while `--profile-lod-work` reports per-LOD filled samples; both are off by default.
+Implementation and evidence: [`Standalone/Research/WorldAssemblerM3/Linux9/REPORT.md`](Standalone/Research/WorldAssemblerM3/Linux9/REPORT.md).
+
 ## 4. The density pipeline (most-edited hot path)
 
 ### 4.1 `GetDensityWithParams` (TunnelNetwork) — VoxelGenerator.cpp:3749