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/modules/annotate/dim_continue.rs

@@ -58,12 +58,9 @@ impl DimContinueCommand {
     /// `definition_point` — where the dim line was placed (defines perpendicular offset).
     /// `rotation` — 0.0 = horizontal dim, PI/2 = vertical dim.
     pub fn from_base(p1: Vec3, p2: Vec3, definition_point: Vec3, rotation: f64) -> Self {
-        // Axis unit vector along the measurement direction.
-        let axis = if rotation.abs() < 0.1 {
-            Vec3::X
-        } else {
-            Vec3::Y
-        };
+        // Axis unit vector along the measurement direction — the base dim's
+        // rotation angle (any angle, incl. a UCS-aligned one), not a world H/V.
+        let axis = Vec3::new(rotation.cos() as f32, rotation.sin() as f32, 0.0);
         // Perpendicular unit vector toward the dim line.
         let perp = Vec3::new(-axis.y, axis.x, 0.0);
         let dim_offset = (definition_point - p1).dot(perp);