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-after-boundary-summary.txt

@@ -0,0 +1,13 @@
+version=1 voxel_um=250000 tiles=2370 camera_m=(-22.0, -19.0, -22.799999237060547) yaw=0.0 pitch=-4.0
+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 15 30819 54860 9176 12102 -29.7482/-28.0275/-25.2555 0.5335/1.1856/1.9265
+1 448 64 94272 163840 32768 131072 -60.0000/-60.0000/-60.0000 0.0000/0.0000/0.0000
+2 320 40 28476 45376 15296 20480 -60.0000/-60.0000/-60.0000 0.0000/0.0000/0.0000
+3 208 48 58599 93736 30968 24576 -60.0000/-60.0000/-60.0000 0.0000/0.0000/0.0000
+4 240 48 105724 164240 27638 22945 -60.0000/-60.0000/-60.0000 0.0000/0.9167/1.8008
+5 144 96 96543 156615 22706 109798 -506.3981/-502.5195/-60.0000 1.7900/110.1491/138.3581
+6 96 48 114212 157583 28194 91871 -506.2913/-503.3460/-411.9775 78.1123/124.2575/136.4059
+7 96 48 142940 234358 31690 95029 -506.1172/-503.3633/-412.8203 90.7064/105.1855/106.9902
+8 306 153 290960 421667 73118 147296 -505.8558/-503.1657/-422.5029 82.1507/93.3314/110.2594
+near_camera_up_tiles (sorted by XY tile-centre distance):
+key=(-2, -2, -4, 1) xy_distance_m=5.39 vertices=1473 triangles=2560 skirt_triangles=512 upward_triangles=2048 z_m=-60.000/-60.000/-60.000 tile_span_p05_p95_m=0.000