wip(assembler): M3 round 2 -- islands get real mass: deep crust, separate tapered roots, arched spans

Pinch cause: SmoothStep taper drove the underside radius to zero at one point. Version 2: depth 0.75x
diameter (thin-pad control at 0.30 fails), 3-4 seeded roots, bounded contour warp + roughness, one
off-route side island, 3 m rock spans. Bounds/route/W1-W8-W16 pass. Open: legacy probe metadata says
25 cm but production legacy is 12.5 cm -- round 3 must fix the import scale contract.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MmSoF64BbRFmk9C3p1VsBJ

Standalone/Research/WorldAssemblerM3/Round2/results/audit.json

@@ -0,0 +1,29 @@
+{
+  "recipe_hash":"5b8f27274bad89ba",
+  "plan_hash":"ac33c9460b16337a",
+  "manifest_hash":"0db30684eaefa75e",
+  "unchanged_existing_hashes":{"m1_round1_manifest":"c87ae279b0e26f53","surfaceworld_round4_manifest":"8933693d9560ba9f"},
+  "geometry_version":2,
+  "owner_voxel_size_cm":12.5,
+  "core_voxel_size_cm":25,
+  "owner_to_core_length_scale":0.5,
+  "plan_islands":4,
+  "plan_connectors":3,
+  "route_points":29,
+  "legacy_anchor_owner_voxels":[48,16,-1904.03515625],
+  "comparison_anchor_core_voxels":[24,8,-952.015625],
+  "legacy_anchor_plateau_samples":25,
+  "recipe_parameters":{"seed":0,"primary_island_count":3,"density_q16":32768,"spacing_voxels":56,"radius_voxels":[12,24],"thickness_ratio_q16":39322,"vertical_jitter_q16":39322,"top_flatten_q16":39322,"contour_warp_voxels":6.30078125,"crust_depth_bonus_q16":9830,"root_depth_ratio_q16":49152,"roughness_voxels":1.5,"blend_voxels":3,"bridge_half_width_voxels":6,"bridge_thickness_voxels":4},
+  "stable_island_ids":["e6e461e18b3a9c87","2bc1820baba047f6","adc259cbe2a395e9","7855fb022cc561c9"],
+  "stable_connector_ids":["eaa5dd310e846520","034023599b466c51","4962fa885545fb26"],
+  "island_profiles":[{"id":"e6e461e18b3a9c87","ordinal":0,"center_voxels":[-15.19921875,25.2734375,-957.5078125],"radius_voxels":14.08203125,"top_voxels":-954.69140625,"bottom_voxels":-975.81640625,"depth_to_diameter":0.750069348128},{"id":"2bc1820baba047f6","ordinal":1,"center_voxels":[24,-12.2265625,-954.8984375],"radius_voxels":14.42578125,"top_voxels":-952.015625,"bottom_voxels":-973.65625,"depth_to_diameter":0.75006769564},{"id":"adc259cbe2a395e9","ordinal":2,"center_voxels":[63.19921875,26.28515625,-958.57421875],"radius_voxels":20.23828125,"top_voxels":-954.52734375,"bottom_voxels":-984.88671875,"depth_to_diameter":0.750048253233},{"id":"7855fb022cc561c9","ordinal":3,"center_voxels":[-1.19921875,-68.2265625,-950.75390625],"radius_voxels":22.8125,"top_voxels":-946.19140625,"bottom_voxels":-980.41015625,"depth_to_diameter":0.75}],
+  "bounds":{"passed":true,"negative_control_detected":true,"boxes":515,"samples":13851,"unknown":74,"missed":0,"island_boxes":4,"connector_boxes":3,"void_boxes_proved_air":1},
+  "thickness":{"passed":true,"islands_tested":4,"islands_below_minimum":0,"minimum_depth_to_diameter_ratio":0.75,"required_depth_to_diameter_ratio":0.7,"minimum_island_id":"7855fb022cc561c9"},
+  "thin_pad_negative_control":{"detected":true,"failed_islands":4,"minimum_depth_to_diameter_ratio":0.299891686975},
+  "route":{"passed":true,"segments":28,"centers":254,"capsule_samples":69088,"blocked":0,"support_samples":1430,"unsupported":0,"grade_failures":0,"minimum_clearance_subvoxels":78},
+  "raised_route_negative_control":{"detected":true,"passed":false,"unsupported":254,"blocked":0},
+  "determinism":{"W1":"566b6115f1aaeb71","W8":"566b6115f1aaeb71","W16":"566b6115f1aaeb71","plan_repeat":true,"hash_samples":32768},
+  "timing":{"same_queries":32768,"legacy_ms":64.6405,"native_ms":38.0581,"native_over_legacy":0.588765557197,"sign_mismatches":1454,"sign_mismatch_percent":4.43725585938,"legacy_solid_samples":130,"native_solid_samples":1380,"legacy_only_solid_samples":102,"native_only_solid_samples":1352},
+  "mesh":[{"name":"cluster_legacy","grid_samples":36613,"triangles":1532,"bytes":73544},{"name":"cluster_native","grid_samples":36613,"triangles":8764,"bytes":420680},{"name":"detail_legacy","grid_samples":207025,"triangles":4124,"bytes":197960},{"name":"detail_native","grid_samples":207025,"triangles":27794,"bytes":1334120}],
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+}