- DocumentTab: bg_color / paper_bg_color Optional<[f32;4]> fields
- view.rs: use custom color when set, fall back to defaults
- BACKGROUND <r> <g> <b>: set model or paper-space background (0–255 RGB)
- BACKGROUND RESET: restore default dark grey / paper grey
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
- Add `active_ucs: Option<Ucs>` per-tab state to DocumentTab
- Add `ucs_to_wcs`, `wcs_to_ucs`, `ucs_z_axis`, `ucs_rotated_z` helpers
- Add `Camera::pick_on_plane` for ray–plane intersection against any plane
- Mouse picks project onto the active UCS XY plane instead of world XY
- Typed coordinates are converted from UCS space to WCS before dispatch
- UCS command now really activates/deactivates a UCS:
- `UCS W` resets to WCS (clears active_ucs)
- `UCS <name>` activates a named UCS
- `UCS SAVE <name>` saves the current active UCS
- `UCS ORIGIN x,y,z` shifts origin (in current UCS space)
- `UCS X/Y/Z <angle>` rotates the UCS around its own axes
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>