core: A/A noise floor (±1%) and small hot-path trims for the cave planner (L137, output = v236)

Identical builds differ <=0.6% median; residual near-ready vs assembler/world is real: Surface World +2.9..+3.6%,
Vertical Shafts +4.0..+4.3%, Tunnel Network -3.6/+6.8%. Profile points at column-cache reads and the sweep lookup.
Removed an unused sample field and per-lookup shared_ptr refcounting. 16/16 seeds, fast tier, Clang/GCC green.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016dhxM43dnWvUu2B5Ph1iG1

Standalone/Research/WorldAssemblerM3/Linux137/REPORT.md

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+# Linux137 report
+
+## Measurement
+
+Two `assembler/world` binaries had identical SHA-256. On Surface World, their L80 near-ready medians differed by +0.52% at W8 and −0.59% at W12. The no-op v228 control differed by −0.37% and −0.05%; its stream hashes matched v227. Use ±1% as the median noise floor. Individual paired controls varied by up to about 2.5% (3.6% for one no-op pair).
+
+The three-run L77 profile sampled LOD 0–8 with no lost samples. Function share changes, `HEAD` versus `assembler/world`: `CachedColumn` +0.93 pp, shared column `TryRead` +0.58 pp, `prepareSweptTubeBase` +0.38 pp, and `sweptSample` +0.17 pp. `ApplyResolvedStratePassageCarves` fell by 0.97 pp and `FindBoundStratumForManagerZ` by 0.39 pp. The profile points more toward column-cache work and the sweep lookup than mouth-profile work; it does not isolate one cause for every world.
+
+Changes retained: removed the unused `floorShellSignedDistance` sample field; removed per-lookup shared-pointer refcounting and batched diagnostic counters per query scope.
+
+## L80 near-ready medians
+
+Each delta is candidate versus `assembler/world`; the A/A median noise floor is ±1%.
+
+| World | W8 delta | W12 delta |
+|---|---:|---:|
+| Surface World | +3.58% (±1%) | +2.90% (±1%) |
+| Floating Islands | +0.19% (±1%) | +2.20% (±1%) |
+| Tunnel Network | −3.60% (±1%) | +6.84% (±1%) |
+| Vertical Shafts | +4.31% (±1%) | +3.96% (±1%) |
+
+The requested targets are not met except Floating Islands W8 and Tunnel Network W8. Whole-view medians were within −8.39% to +0.27% of baseline.
+
+## Verification
+
+Fast tier passed (273 checks); the composed sweep passed 16/16 seeds with seams and blocked controls; Clang/GCC W1/W8/W12 matched all 13 hashes. Linux UE host build passed with `-NoUBA -MaxParallelActions=4` and no `prlimit`. Output audits stayed green, so version and baselines were left unchanged. No commit, push, or stash.