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/mesh-sw-after-summary.txt

@@ -0,0 +1,12 @@
+version=1 voxel_um=250000 tiles=2553 camera_m=(0.0, -35.0, -487.94000244140625) yaw=0.0 pitch=-22.0
+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
+0 512 0 0 0 0 0 nan/nan/nan nan/nan/nan
+1 448 0 0 0 0 0 nan/nan/nan nan/nan/nan
+2 448 0 0 0 0 0 nan/nan/nan nan/nan/nan
+3 384 0 0 0 0 0 nan/nan/nan nan/nan/nan
+4 320 52 139437 209316 34996 8129 -56.2002/-42.7865/-32.7998 6.2138/32.5482/53.6770
+5 144 102 100254 158707 25586 96013 -506.9092/-503.1813/-378.0397 12.8110/109.5153/138.3581
+6 96 61 110399 149031 17088 92803 -506.4873/-503.3776/-412.3926 53.2054/124.2575/136.4059
+7 48 48 132222 219052 16356 95003 -506.1172/-503.3639/-412.8861 90.7064/105.1855/106.9902
+8 153 153 229080 384768 82518 147226 -505.8568/-503.1676/-422.8789 82.0687/93.3180/110.0400
+near_camera_up_tiles (sorted by XY tile-centre distance):