Add button to subdivide the next N worst cells

[ajlennon]

Summary

• Interactive cell splits are no longer all-or-nothing after bake: Subdivide next N cells (default N=10) walks the worst out-of-tolerance cells one batch at a time (Julian CNC 149).
• Build still does bar refine; auto cell refine stays at 0 in the editor, and headless/CI keeps max_refine_passes so quality gates are unchanged.

Test plan

• Build tool surface on main mesh; confirm no bulk cell splits until the button is pressed
• Press Subdivide next N cells with N=10; status shows how many split; lattice updates
• Press again until status says no more out-of-tolerance cells
• CI eartip/drop quality still green

Assisted-by: Cursor cursoragent@cursor.com