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SMLib's flat arrays now carry the corner of three equal rounds as `SURFACE_BALL_PATCH` (kind 6): its net as a NURBS face's block, with its sphere's centre and radius as its definition (ids 36 and 166). The record reads it as `ball_patch {center, radius}`, a label like the analytic kinds, with the net as its `nurbs`. A plain sphere with a form won't do: the net is not a piece of SMLib's sphere NURBS, so SMLib's `IsNurbSurfaceSphere` would refuse it.
Written back, a ball patch is kind 6 with the same definition, which SMLib takes once its net is certified on that sphere. Elsewhere:
- Offsets classify it as its sphere, so its offsets stay exact and collapse at the centre.
- Face kinds count it as a sphere: fillet_three_edges_at_a_corner's corner is now `sphere`, no longer `other`.
- STEP, which has no ball-patch entity, gets the net as the face's B-spline surface, pcurves included.
- Iso curves and SSI of the label are refused; the face's form is what evaluates.
Test: a_rounded_cubes_ball_patches_go_round_with_their_spheres. The cube's eight corners are ball patches about the corners pulled in by r. Points of each patch lie on its sphere to 1e-12. The patches are kind 6 again after import, with the same definitions bit for bit, and the closed-form volume holds.
Format: pending/ball-patch-faces.rs. Docs: docs/native-brep.md and the rig README's `surface_types` (`ball_patch`).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DtL5M41G8Nua7HvuFPZkmJ