wip(assembler): M1 round 2 -- surface recipe closer to legacy; native still slower and visibly buggy

Luna, M1 round 2 of 4: round 1's mismatch was an underfit recipe (two synthetic profiles,
missing large-scale surface terms). Now: biome IDs match 4,096/4,096, mean height offset
-1.366 vox, density MAE 274.04 -> 17.23 vox, sign mismatches 55.2% -> 18.14%. Bound audit
298 boxes / 8,046 points, 0 misses, negative control caught. Hash c5e0371612e927af at
W1/W8/W16. Surface query 8-14 ms / 4,096 points. Matched 3D capture pairs added.
Problems (my review of the captures + her timings):
- matched 120-tile views: full-detail readiness 5.227 s native vs 2.882 s legacy -- SLOWER;
- surface capture shows a rectangular flat-topped raised block with vertical walls
  (tile/patch-level height bug, inferred);
- cliff capture is broken (floating shard, debris trail), not a cliff.

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

Standalone/Research/WorldAssemblerM1/round2/world_assembly.json

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