feat(ucs): central WCS↔UCS converter; adopt file UCS, drive readout + icon

Introduce one bridge between WCS (how geometry and the file are stored)
and the active UCS (the coordinate system the user works in), so every
UCS-aware system routes through it instead of re-deriving axis math.

- UcsXform (app/helpers.rs): the converter — to_wcs/to_ucs (points),
  vec_to_wcs/vec_to_ucs (directions), axes(), is_identity(). Orthonormal
  axes, so the inverse is the transpose. The old ucs_to_wcs / ucs_rotate_vec
  free fns now delegate to it (one implementation).
- DocumentTab::ucs_xform() builds it from the tab's active_ucs (None = WCS).
- DocumentTab::adopt_active_ucs_from_header() loads the document's saved
  current UCS (header model-space UCS) into active_ucs on open, so the file's
  coordinate system is live immediately; called from both load paths.
- Status-bar coordinate readout reports the cursor in the active UCS.
- UCS icon tripod projects the active UCS axes, so it rotates to the UCS
  instead of always showing world X/Y/Z.

First slice of routing the app's coordinate systems through the converter;
input/picking, grid, ViewCube, and the remaining readouts follow.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update.rs

@@ -560,6 +560,8 @@ impl OpenCADStudio {
 
                 self.tabs[i].current_path = Some(path.clone());
                 self.tabs[i].scene.document = doc;
+                // Follow the file's saved current UCS from the moment it opens.
+                self.tabs[i].adopt_active_ucs_from_header();
                 // Route shared CJK ideographs to the language matching this
                 // drawing's code page (web per-language font split). Drop the
                 // glyph cache if it changed so Han re-resolves to the new