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/RingProbe.py

@@ -0,0 +1,151 @@
+#!/usr/bin/env python3
+"""Compare upward mesh heights on shared XY edges between adjacent LOD rings."""
+import array
+import collections
+import math
+import struct
+import sys
+
+
+def read_scene(path):
+    tiles = []
+    with open(path, "rb") as f:
+        if f.read(8) != b"VFMSH001":
+            raise SystemExit(f"{path}: invalid mesh header")
+        version, voxel_um, count = struct.unpack("<3I", f.read(12))
+        camera = struct.unpack("<3d", f.read(24))
+        yaw, pitch = struct.unpack("<2d", f.read(16))
+        voxel_m = voxel_um / 1_000_000.0
+        for _ in range(count):
+            key = struct.unpack("<4i", f.read(16))
+            surface_vertices, skirt_count, vertex_count, index_count, mask_count = struct.unpack(
+                "<5I", f.read(20))
+            positions = array.array("f")
+            positions.frombytes(f.read(vertex_count * 12))
+            normals = array.array("f")
+            normals.frombytes(f.read(vertex_count * 12))
+            indices = array.array("I")
+            indices.frombytes(f.read(index_count * 4))
+            masks = f.read(mask_count)
+            if sys.byteorder != "little":
+                positions.byteswap(); normals.byteswap(); indices.byteswap()
+            triangles = []
+            for i in range(0, index_count, 3):
+                if masks and masks[i // 3] != 0:
+                    continue
+                a, b, c = indices[i:i + 3]
+                if max(a, b, c) >= surface_vertices:
+                    continue
+                nz = (normals[a * 3 + 2] + normals[b * 3 + 2] + normals[c * 3 + 2]) / 3.0
+                if nz < 0.65:
+                    continue
+                tri = tuple((positions[j * 3], positions[j * 3 + 1], positions[j * 3 + 2])
+                            for j in (a, b, c))
+                triangles.append(tri)
+            if triangles:
+                lod = key[3]
+                extent = 32 * (1 << lod) * voxel_m
+                tiles.append({"key": key, "lod": lod, "extent": extent,
+                              "x0": key[0] * extent, "y0": key[1] * extent,
+                              "x1": (key[0] + 1) * extent, "y1": (key[1] + 1) * extent,
+                              "z0": key[2] * extent, "z1": (key[2] + 1) * extent,
+                              "triangles": triangles})
+    return camera, yaw, pitch, tiles
+
+
+def height_at(tile, x, y):
+    best = None
+    for tri in tile["triangles"]:
+        (x0, y0, z0), (x1, y1, z1), (x2, y2, z2) = tri
+        den = (y1 - y2) * (x0 - x2) + (x2 - x1) * (y0 - y2)
+        if abs(den) < 1.0e-10:
+            continue
+        a = ((y1 - y2) * (x - x2) + (x2 - x1) * (y - y2)) / den
+        b = ((y2 - y0) * (x - x2) + (x0 - x2) * (y - y2)) / den
+        c = 1.0 - a - b
+        if min(a, b, c) >= -1.0e-5:
+            z = a * z0 + b * z1 + c * z2
+            if best is None or z > best:
+                best = z
+    return best
+
+
+path = sys.argv[1]
+camera, yaw, pitch, tiles = read_scene(path)
+by_lod = collections.defaultdict(list)
+for tile in tiles:
+    by_lod[tile["lod"]].append(tile)
+print(f"scene={path} camera_m={camera} yaw={yaw} pitch={pitch} nonempty_mesh_tiles={len(tiles)}")
+print("lod_pair shared_edges samples_both median_abs_cm p95_abs_cm max_abs_cm median_delta_cm gaps")
+for lod in range(8):
+    fine = by_lod[lod]
+    coarse = by_lod[lod + 1]
+    deltas = []
+    edge_rows = []
+    edge_count = 0
+    gap_count = 0
+    # Shared XY edges occur only where tile extents touch and overlap on the other axis.
+    for a in fine:
+        for b in coarse:
+            zlo, zhi = max(a["z0"], b["z0"]), min(a["z1"], b["z1"])
+            if zhi - zlo <= 1.0e-4:
+                continue
+            edge = None
+            if abs(a["x1"] - b["x0"]) < 1.0e-4 or abs(b["x1"] - a["x0"]) < 1.0e-4:
+                lo, hi = max(a["y0"], b["y0"]), min(a["y1"], b["y1"])
+                if hi - lo > 1.0e-4:
+                    edge = ("x", a["x1"] if abs(a["x1"] - b["x0"]) < 1.0e-4 else a["x0"], lo, hi)
+            if edge is None and (abs(a["y1"] - b["y0"]) < 1.0e-4 or abs(b["y1"] - a["y0"]) < 1.0e-4):
+                lo, hi = max(a["x0"], b["x0"]), min(a["x1"], b["x1"])
+                if hi - lo > 1.0e-4:
+                    edge = ("y", a["y1"] if abs(a["y1"] - b["y0"]) < 1.0e-4 else a["y0"], lo, hi)
+            if edge is None:
+                continue
+            edge_count += 1
+            axis, fixed, lo, hi = edge
+            epsilon = min(0.02, a["extent"] * 0.001, b["extent"] * 0.001)
+            samples = 9
+            matched = 0
+            edge_deltas = []
+            for i in range(samples):
+                t = (i + 0.5) / samples
+                varying = lo + (hi - lo) * t
+                if axis == "x":
+                    side_a = 1.0 if abs(a["x1"] - fixed) < 1.0e-4 else -1.0
+                    side_b = 1.0 if abs(b["x1"] - fixed) < 1.0e-4 else -1.0
+                    za = height_at(a, fixed - side_a * epsilon, varying)
+                    zb = height_at(b, fixed - side_b * epsilon, varying)
+                else:
+                    side_a = 1.0 if abs(a["y1"] - fixed) < 1.0e-4 else -1.0
+                    side_b = 1.0 if abs(b["y1"] - fixed) < 1.0e-4 else -1.0
+                    za = height_at(a, varying, fixed - side_a * epsilon)
+                    zb = height_at(b, varying, fixed - side_b * epsilon)
+                if za is not None and not zlo - 1.0e-3 <= za <= zhi + 1.0e-3:
+                    za = None
+                if zb is not None and not zlo - 1.0e-3 <= zb <= zhi + 1.0e-3:
+                    zb = None
+                if za is not None and zb is not None:
+                    delta = (za - zb) * 100.0
+                    deltas.append(delta)
+                    edge_deltas.append((delta, za, zb, varying))
+                    matched += 1
+            if matched == 0:
+                gap_count += 1
+            elif edge_deltas:
+                edge_rows.append((max(abs(row[0]) for row in edge_deltas),
+                                  a["key"], b["key"], edge, edge_deltas))
+    if deltas:
+        absolute = sorted(abs(x) for x in deltas)
+        signed = sorted(deltas)
+        p95 = absolute[math.floor(0.95 * (len(absolute) - 1))]
+        median_abs = absolute[len(absolute) // 2]
+        median_delta = signed[len(signed) // 2]
+        print(f"{lod}->{lod+1} {edge_count} {len(deltas)} {median_abs:.3f} {p95:.3f} {max(absolute):.3f} {median_delta:.3f} {gap_count}")
+        for worst, fine_key, coarse_key, edge, rows in sorted(edge_rows, reverse=True)[:2]:
+            row = max(rows, key=lambda item: abs(item[0]))
+            print(f"  worst_edge max_abs_cm={worst:.3f} fine={fine_key} coarse={coarse_key} "
+                  f"axis={edge[0]} fixed_m={edge[1]:.3f} span_m={edge[2]:.3f}..{edge[3]:.3f} "
+                  f"sample_v_m={row[3]:.3f} fine_z_m={row[1]:.3f} "
+                  f"coarse_z_m={row[2]:.3f} delta_cm={row[0]:.3f}")
+    else:
+        print(f"{lod}->{lod+1} {edge_count} 0 n/a n/a n/a n/a {gap_count}")