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/MeshProbe.py

@@ -0,0 +1,100 @@
+#!/usr/bin/env python3
+"""Summarize exported tile geometry and upward-facing ground for L17."""
+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]
+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, "vertices": 0,
+        "triangles": 0, "surface_triangles": 0, "up_triangles": 0,
+        "up_centroids": [], "tile_spans": [], "empty": 0})
+    tile_rows = []
+    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]
+        row = per_lod[lod]
+        row["tiles"] += 1
+        row["vertices"] += vertex_count
+        row["triangles"] += index_count // 3
+        row["surface_triangles"] += index_count // 3 - skirt_count
+        up = []
+        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
+            if (n[a * 3 + 2] + n[b * 3 + 2] + n[c * 3 + 2]) / 3.0 < 0.65:
+                continue
+            up.append((p[a * 3 + 2] + p[b * 3 + 2] + p[c * 3 + 2]) / 3.0)
+        row["up_triangles"] += len(up)
+        row["up_centroids"].extend(up)
+        if up:
+            up.sort()
+            span = up[math.floor(0.95 * (len(up) - 1))] - up[math.floor(0.05 * (len(up) - 1))]
+            row["tile_spans"].append(span)
+            tile_rows.append((key, vertex_count, index_count // 3, skirt_count,
+                              len(up), up[0], up[len(up) // 2], up[-1], span))
+        else:
+            row["empty"] += 1
+            tile_rows.append((key, vertex_count, index_count // 3, skirt_count,
+                              0, math.nan, math.nan, math.nan, math.nan))
+
+print(f"version={version} voxel_um={voxel_um} tiles={tile_count} camera_m={camera} yaw={yaw} pitch={pitch}")
+print("lod tiles mesh_tiles vertices triangles skirt_triangles upward_surface_triangles "
+      "upward_centroid_z_m_p05/median/p95 tile_up_z_span_m_p50/p95/max")
+for lod in sorted(per_lod):
+    r = per_lod[lod]
+    z = sorted(r["up_centroids"])
+    s = sorted(r["tile_spans"])
+    def q(values, fraction):
+        return values[math.floor(fraction * (len(values) - 1))] if values else math.nan
+    print(f"{lod} {r['tiles']} {r['tiles']-r['empty']} {r['vertices']} {r['triangles']} "
+          f"{sum(row[3] for row in tile_rows if row[0][3] == lod)} {r['up_triangles']} "
+          f"{q(z,.05):.4f}/{q(z,.5):.4f}/{q(z,.95):.4f} "
+          f"{q(s,.5):.4f}/{q(s,.95):.4f}/{max(s) if s else math.nan:.4f}")
+
+print("near_camera_up_tiles (sorted by XY tile-centre distance):")
+near = []
+for row in tile_rows:
+    key = row[0]
+    lod = key[3]
+    extent = 32 * (1 << lod) * voxel_um / 1_000_000.0
+    cx = (key[0] + 0.5) * extent
+    cy = (key[1] + 0.5) * extent
+    near.append((math.hypot(cx-camera[0], cy-camera[1]), row))
+for distance, row in sorted(near)[:30]:
+    key, verts, tris, skirts, ups, lo, med, hi, span = row
+    if ups:
+        print(f"key={key} xy_distance_m={distance:.2f} vertices={verts} triangles={tris} "
+              f"skirt_triangles={skirts} upward_triangles={ups} z_m={lo:.3f}/{med:.3f}/{hi:.3f} "
+              f"tile_span_p05_p95_m={span:.3f}")