wip(L17, side branch): FloatingIslands seal-cap fix + owner/Core chunk-scale change (breaks SurfaceWorld)

Not for assembler/world: the 8 m -> 4 m strate chunk change moves every stratum, so the SurfaceWorld
spawn camera floats in a void and the L1 mesh export shrank 58.7 -> 31.7 MB. Kept for its
diagnostics, the boundarySeal finding and the readable capture shading. See Linux17/REPORT.md.

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

Standalone/Research/WorldAssemblerM3/Linux17/BoundaryProbe.py

@@ -0,0 +1,89 @@
+#!/usr/bin/env python3
+"""Measure upward mesh faces around the FI/SW stratum interface."""
+import array
+import collections
+import math
+import struct
+import sys
+
+
+def read_exact(stream, size):
+    data = stream.read(size)
+    if len(data) != size:
+        raise EOFError(f"wanted {size} bytes, got {len(data)}")
+    return data
+
+
+path = sys.argv[1]
+low_z = float(sys.argv[2]) if len(sys.argv) > 2 else -80.0
+high_z = float(sys.argv[3]) if len(sys.argv) > 3 else -45.0
+with open(path, "rb") as stream:
+    if read_exact(stream, 8) != b"VFMSH001":
+        raise SystemExit("bad VFMSH001 header")
+    version, voxel_um, tile_count = struct.unpack("<3I", read_exact(stream, 12))
+    camera = struct.unpack("<3d", read_exact(stream, 24))
+    yaw, pitch = struct.unpack("<2d", read_exact(stream, 16))
+    per_lod = collections.defaultdict(lambda: {"tiles": 0, "faces": [], "spans": []})
+    near_tiles = collections.defaultdict(list)
+    for _ in range(tile_count):
+        key = struct.unpack("<4i", read_exact(stream, 16))
+        surf_vertices, skirt_count, vertex_count, index_count, mask_count = struct.unpack(
+            "<5I", read_exact(stream, 20))
+        p = array.array("f")
+        p.frombytes(read_exact(stream, vertex_count * 12))
+        n = array.array("f")
+        n.frombytes(read_exact(stream, vertex_count * 12))
+        if sys.byteorder != "little":
+            p.byteswap(); n.byteswap()
+        indices = array.array("I")
+        indices.frombytes(read_exact(stream, index_count * 4))
+        masks = read_exact(stream, mask_count)
+        if sys.byteorder != "little":
+            indices.byteswap()
+        lod = key[3]
+        z_values = []
+        for tri in range(index_count // 3):
+            if masks and masks[tri] != 0:
+                continue
+            a, b, c = indices[tri * 3:tri * 3 + 3]
+            if max(a, b, c) >= surf_vertices:
+                continue
+            normal_z = (n[a * 3 + 2] + n[b * 3 + 2] + n[c * 3 + 2]) / 3.0
+            if normal_z < 0.65:
+                continue
+            z = (p[a * 3 + 2] + p[b * 3 + 2] + p[c * 3 + 2]) / 3.0
+            if low_z <= z <= high_z:
+                z_values.append(z)
+        if not z_values:
+            continue
+        z_values.sort()
+        row = per_lod[lod]
+        row["tiles"] += 1
+        row["faces"].extend(z_values)
+        span = z_values[math.floor(.95 * (len(z_values) - 1))] - z_values[math.floor(.05 * (len(z_values) - 1))]
+        row["spans"].append(span)
+        extent = 32 * (1 << lod) * voxel_um / 1_000_000.0
+        cx = (key[0] + 0.5) * extent
+        cy = (key[1] + 0.5) * extent
+        near_tiles[lod].append((math.hypot(cx - camera[0], cy - camera[1]), key,
+                                z_values[len(z_values) // 2], span, len(z_values)))
+
+
+def q(values, fraction):
+    return values[math.floor(fraction * (len(values) - 1))] if values else math.nan
+
+
+print(f"camera_m={camera} yaw={yaw} pitch={pitch} boundary_band_m={low_z}..{high_z}")
+print("lod tiles_with_boundary_faces faces z_m_p05/median/p95 tile_span_m_p50/p95/max")
+for lod in sorted(per_lod):
+    row = per_lod[lod]
+    spans = sorted(row["spans"])
+    print(f"{lod} {row['tiles']} {len(row['faces'])} "
+          f"{q(sorted(row['faces']), .05):.4f}/{q(sorted(row['faces']), .5):.4f}/"
+          f"{q(sorted(row['faces']), .95):.4f} "
+          f"{q(spans, .5):.4f}/{q(spans, .95):.4f}/{max(spans):.4f}")
+print("nearest_xy_boundary_tiles_by_lod:")
+for lod in sorted(near_tiles):
+    for distance, key, median, span, faces in sorted(near_tiles[lod])[:3]:
+        print(f"lod={lod} key={key} xy_distance_m={distance:.2f} "
+              f"boundary_median_z={median:.4f} within_tile_p05_p95_m={span:.4f} faces={faces}")