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

[trailcode]

Summary

Follow-up polish for the Add bone sketch tool (landed in #256): move bone geometry into skt_bone, add optional DEV_MODE construction overlays, and reverse end-radius placement so the first radius click is at the second center (where the cursor already is).

Problem

• Bone outline/waist math lived in utl_geom, mixed with general geometry helpers.
• Construction geometry (cutters, capsule tangents, contacts) was hard to inspect while placing a bone.
• After placing the two centers, radius entry started at the first center while the mouse was still near the second.

Implemented scope

• Extract bone geom into skt_bone.h / skt_bone.cpp (compute_bone_geom, make_bone_wire, profile/debug shapes).
• Compile-time DEV_MODE in config.h; Add-bone Options Debug vis checkboxes (session-only).
• Place radius 2 then radius 1 after centers; update live preview, user guide, and CHANGELOG.
• Vertical-rhythm style note + code_style_check.py for blank line after early-return if.

Acceptance criteria

• Add bone: centers -> radius at second center -> radius at first center -> waist (and holes if enabled).
• With DEV_MODE=1, Debug vis overlays toggle without persisting settings.
• Committed outline unchanged aside from placement UX; geom still solves from waist.
• Docs (usage-sketch.md, src/doc/sketch.md / gui.md, CHANGELOG) match behavior.

Related

• Prior: Add bone sketch tool; extrude Both sides default; wasm CMake fixes #256 Add bone sketch tool
• PR: Add bone follow-ups: skt_bone, DEV_MODE debug vis, reverse radius order #260