Add bone follow-ups: skt_bone, DEV_MODE debug vis, reverse radius order

[trailcode]

Summary

• Extract bone outline/waist geometry from utl_geom into skt_bone (compute_bone_geom, make_bone_wire, debug overlays).
• Add compile-time DEV_MODE and Add-bone Options Debug vis checkboxes (session-only construction overlays).
• Reverse end-radius placement: after centers, size the second circle first (cursor is already there), then the first.
• Style: blank line after early-return if; code_style_check.py + docs.

Closes #259

Test plan

• Add bone: centers -> radius at second center -> radius at first -> waist (holes if enabled)
• DEV_MODE=1: Debug vis toggles (capsule tangents, cutters, contacts, radials) update overlay
• Commit outline looks correct; Tab still enters exact radii/waist
• Spot-check docs/usage-sketch.md and CHANGELOG
• Build EzyCad_lib / EzyCad_tests