wip(assembler): round L31 -- FI recipe column audit corrected; recipe-stamped audit runner

On the real FloatingIslands recipe, column caps sit inside anchor-island rock and native bedrock;
the old check sampled the route foot. The audit now samples each planned cap against the native
field (negative control: a cap moved into air is detected). The audit runner records requested vs
actual recipe and exits on mismatch. All 16 passage rows (4 styles x 2 recipes x 2 compilers) pass.
The readable shader uses a smooth tint. Interim: FI Gradient displacement capped at width/256
(flat floor, no contour rings); restoring real undulation is L32. Evaluator 19.

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

Standalone/Research/WorldAssemblerM3/Linux31/AggregateCostL31.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+"""Combine the initial and repeat L31 interleaved cost batches."""
+import csv
+import statistics
+from pathlib import Path
+
+
+ROOT = Path(__file__).resolve().parents[4]
+OUT = ROOT / "Saved/Linux/l31"
+sources = (
+    ("first", 0, OUT / "cost-first/cost-runs.csv"),
+    ("repeat", 5, OUT / "cost-runs.csv"),
+)
+rows = []
+for batch, offset, path in sources:
+    with path.open(newline="", encoding="utf-8") as stream:
+        for row in csv.DictReader(stream):
+            row["pair"] = str(int(row["pair"]) + offset)
+            row["source_batch"] = batch
+            rows.append(row)
+
+with (OUT / "cost-combined.csv").open("w", newline="", encoding="utf-8") as stream:
+    writer = csv.DictWriter(stream, fieldnames=(
+        "recipe", "pair", "build", "whole_view_ready_seconds", "process_elapsed_seconds",
+        "exit_code", "status", "source_batch"))
+    writer.writeheader()
+    writer.writerows(rows)
+
+print("recipe pairs median_l30_s median_l31_s median_change_pct paired_median_pct range_pct")
+failed = False
+for recipe in ("surface-world", "floating-islands"):
+    by_build = {
+        build: [float(row["whole_view_ready_seconds"]) for row in rows
+                if row["recipe"] == recipe and row["build"] == build]
+        for build in ("l30", "l31")
+    }
+    paired = []
+    for pair in range(1, 11):
+        times = {
+            row["build"]: float(row["whole_view_ready_seconds"])
+            for row in rows if row["recipe"] == recipe and int(row["pair"]) == pair
+        }
+        paired.append((times["l31"] / times["l30"] - 1.0) * 100.0)
+    median_l30 = statistics.median(by_build["l30"])
+    median_l31 = statistics.median(by_build["l31"])
+    median_change = (median_l31 / median_l30 - 1.0) * 100.0
+    print(f"{recipe} 10 {median_l30:.3f} {median_l31:.3f} {median_change:+.2f} "
+          f"{statistics.median(paired):+.2f} {min(paired):+.2f}..{max(paired):+.2f}")
+    failed |= len(by_build["l30"]) != 10 or len(by_build["l31"]) != 10
+print(f"[L31CostCombined] pairs_per_recipe=10 runs={len(rows)} "
+      f"status={'FAIL' if failed else 'PASS'} summary={OUT / 'cost-combined.csv'}")
+raise SystemExit(1 if failed or len(rows) != 40 else 0)