feat(ucs): orient text-bearing annotations to the active UCS

New annotations now place their text and reading geometry along the
active UCS X axis (identity = world), so they read square to the user's
coordinate system instead of world-horizontal:

- TEXT / MTEXT: rotation defaults to the UCS X angle
  (DocumentTab::ucs_rotation_angle, model space only).
- Linear dimensions: measure along the UCS axis the span is nearest, and
  bake the UCS X angle as the text rotation so the value reads square
  even under a style that forces horizontal text. DIMCONTINUE / DIMBASELINE
  derive their axis from the base dim's rotation (any angle), not a
  world H/V test.
- LEADER: the MText annotation rotates to the UCS, and the hookline runs
  along the leader's horizontal_direction (set to UCS X), used by the
  renderer instead of world horizontal.
- MLEADER: context.text_direction = UCS X drives both the text rotation
  and, in the renderer, the text side and dogleg/landing direction.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/text_inline.rs

@@ -225,6 +225,8 @@ impl super::OpenCADStudio {
             if !cur_style.is_empty() {
                 t.style = cur_style;
             }
+            // Align new text to the active UCS (baseline along the UCS X axis).
+            t.rotation = self.tabs[i].ucs_rotation_angle();
             self.push_undo_snapshot(i, "TEXT");
             self.commit_entity(EntityType::Text(t));
             self.tabs[i].dirty = true;