Add permanent sketch origin node per sketch

[trailcode]

Closes #175

Summary

• Adds a built-in Origin node to every sketch, shown as a permanent red + marker.
• Reference-plane sketches: origin at (0, 0) on the sketch plane.
• Face-derived sketches: origin at the bounding-box center of the outer face wire.
• Origin is permanent, snap-enabled, non-deletable, and persisted in JSON (origin on the node).
• Fixes origin being removed on mode change by calling m_nodes.finalize() after committing the origin.
• Syncs permanent node marks when a sketch becomes current.

Documentation

• Developer: src/doc/sketch.md
• User guide: new Sketch origin section, cross-links from usage.md, usage-settings.md
• Settings ? help for permanent node marker scale links to sketch origin
• CHANGELOG.md [Unreleased] entry

Test plan

• SketchOriginNodePlane — origin at plane (0, 0)

• SketchOriginNodeFromFaceBBoxCenter — origin at face wire bbox center

• SketchOriginNodeSurvivesOnMode — origin not wiped by on_mode() / cancel_elm()

• Full Sketch_test suite passes

• Manual: create sketch from planar face; confirm red + at face center

• Manual: create sketch on XY plane; confirm + at plane origin

• Manual: try deleting Origin via selection (should not remove)

[cursor]

Bugbot is not enabled for your account, so this pull request was not reviewed.

Enable Bugbot in the Cursor dashboard to get automatic reviews on future PRs.