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 <[email protected]>

src/app/document.rs

@@ -217,6 +217,17 @@ impl DocumentTab {
         )
     }
 
+    /// World-space rotation angle (radians) of the active UCS X axis — the
+    /// default rotation for new text-bearing objects so their text aligns to
+    /// the user's coordinate system. Zero outside model space / with no UCS.
+    pub(super) fn ucs_rotation_angle(&self) -> f64 {
+        if self.scene.current_layout != "Model" {
+            return 0.0;
+        }
+        let (_, x, ..) = self.ucs_xform().axes();
+        (x.y as f64).atan2(x.x as f64)
+    }
+
     /// Grid origin (render/wire space) and UCS→world rotation for grid snap and
     /// the grid overlay. Identity / origin-at-zero outside model space.
     pub(super) fn ucs_grid_basis(&self) -> (glam::Vec3, glam::Mat4) {

src/app/mtext_editor.rs

@@ -632,7 +632,7 @@ impl super::OpenCADStudio {
         let i = self.active_tab;
         let Some(ed) = self.mtext_editor.take() else { return false };
         let body_empty = ed.content.text().trim().is_empty();
-        let mt = ed.build_mtext();
+        let mut mt = ed.build_mtext();
         if body_empty {
             // Empty content: drop a new entity; leave an edited one untouched.
             self.refresh_properties();
@@ -661,6 +661,8 @@ impl super::OpenCADStudio {
             self.tabs[i].scene.bump_geometry();
             self.tabs[i].dirty = true;
         } else {
+            // Align new MText to the active UCS (text runs along the UCS X axis).
+            mt.rotation = self.tabs[i].ucs_rotation_angle();
             self.push_undo_snapshot(i, "MTEXT");
             self.commit_entity(EntityType::MText(mt));
             self.tabs[i].dirty = true;

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;