wip(assembler): round L32 -- smooth Gradient interface at full amplitude (no rings, not flat)

Band faces now use signed distance to the displaced solid intervals, continuous across bands, so
MC interpolates the interface instead of terracing at density levels. The Gradient amplitude is
restored to width/8 (about 4 m peak-to-peak at 16 m). Ring/terrace audit at LOD0-8 on the FI floor and SW
ceiling passes; negative controls: L30 (rings) and L31 (flat) both fail. Hole/interface audits,
16 passage rows and 24/24 determinism pass. Evaluator 20. Cost vs L31 SW +1.9%, FI +1.5%.

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

Standalone/Research/WorldAssemblerM3/Linux32/FindSurfaceWorldAirL32.py

@@ -0,0 +1,51 @@
+#!/usr/bin/env python3
+"""Find open SurfaceWorld columns below the resolved transition ceiling."""
+import argparse
+import csv
+import subprocess
+from pathlib import Path
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("probe")
+    parser.add_argument("owner")
+    parser.add_argument("output")
+    parser.add_argument("--z", type=float, default=-130.0)
+    parser.add_argument("--minimum", type=float, default=-96.0)
+    parser.add_argument("--maximum", type=float, default=96.0)
+    parser.add_argument("--spacing", type=float, default=16.0)
+    args = parser.parse_args()
+    probe = str(Path(args.probe).resolve())
+    owner = str(Path(args.owner).resolve())
+    output = Path(args.output)
+    output.parent.mkdir(parents=True, exist_ok=True)
+    rows = []
+    count = round((args.maximum - args.minimum) / args.spacing)
+    for iy in range(count + 1):
+        y = args.minimum + iy * args.spacing
+        for ix in range(count + 1):
+            x = args.minimum + ix * args.spacing
+            command = ["taskset", "-c", "0-11", "prlimit", "--as=6442450944", "--",
+                       probe, owner, "surface-world", str(x), str(y),
+                       str(args.z - 2.0), str(args.z + 2.0), "0.25"]
+            result = subprocess.run(command, capture_output=True, text=True, timeout=1800,
+                                    check=False)
+            samples = [line for line in result.stdout.splitlines()
+                       if line.startswith(f"{args.z:.6f},")]
+            density = float(samples[0].split(",")[1]) if result.returncode == 0 and samples else float("nan")
+            rows.append((x, y, args.z, density, result.returncode))
+            print(f"[SurfaceWorldAirScan] x_m={x:.1f} y_m={y:.1f} z_m={args.z:.2f} "
+                  f"density_vox={density:.6f} exit={result.returncode}", flush=True)
+    with output.open("w", newline="", encoding="utf-8") as stream:
+        writer = csv.writer(stream)
+        writer.writerow(("x_m", "y_m", "z_m", "density_vox", "exit_code"))
+        writer.writerows(rows)
+    open_samples = [row for row in rows if row[3] > 0.0]
+    print(f"[SurfaceWorldAirScanSummary] samples={len(rows)} open={len(open_samples)} "
+          f"output={output} status={'PASS' if open_samples else 'FAIL'}")
+    return 0 if open_samples else 1
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())