wip(standalone): GPU step 20 -- detail band batched by 8 columns; gap instrumented

ApplyDetailBand claims eight adjacent columns per batch, keeping each column's scalar
order: shape stage ~586 -> ~566 ms in Luna's profile. Hashes w1/w8 identical.
My runs outside the sandbox (Step20.exe, marker removed first, interleaved):
  CPU 1.319 / 1.287 / 1.288 s
  GPU 1.320 (marker miss, 0 GPU tiles) / 1.244 / 1.215 / 1.219 s (match, ~1,400 GPU)
Shape compile 549-570 ms in both modes. The 669-821 ms gap in cached runs is still
there: export starts only after [ChunkCrossingWork] reports the ~840 CPU tiles
(LOD0-2) finished, so GPU export waits behind CPU meshing on the shared workers.
Slowest GPU tile 1.7 / 18.4 / 27.3 ms in runs b/c/d.
Audits: hashes, walk 5,642/5,642, near 960 / far 1,114 0 diffs vs CPU and WARP,
thin 353, survival 6.14 GB, ten captures as step 18. Far vertex-distance diagnostic:
247 bound failures (max 64.56 cm), unchanged since step 16.

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

Standalone/Core/src/ShapeWorldInternal.h

@@ -3390,7 +3390,9 @@ private:
             CandidateCount.fetch_add(LocalCandidates,std::memory_order_relaxed);
             ProxyCount.fetch_add(LocalProxies,std::memory_order_relaxed);
             FlipCount.fetch_add(LocalFlips,std::memory_order_relaxed);
-        },&Timing,&WorkerActiveSeconds);
+        // Each X/Y column is independent. Claim eight adjacent columns at a time to
+        // reduce scheduler atomics while preserving per-column scalar evaluation order.
+        },&Timing,&WorkerActiveSeconds,8);
         Stats.SurfaceCandidates=CandidateCount.load(std::memory_order_relaxed);
         Stats.SurfaceProxyNodes=ProxyCount.load(std::memory_order_relaxed);
         Stats.DetailFlips=FlipCount.load(std::memory_order_relaxed);