feat: annotation scale system (CANNOSCALE, PSLTSCALE, MultiLeader, Leader)

- Scale popup dropdown replaces cycle behavior; paper-space viewport scale
  editable when a viewport is active/selected
- CANNOSCALE persisted via DXF $USERR1 (read on open, write on save)
- PSLTSCALE: linetype dash lengths scaled by viewport anno_scale so dashes
  appear uniform physical size in paper space
- MultiLeader: arrow size, dogleg length, and text height scale with
  anno_scale; leader line paths (world coords) stay fixed
- Leader: arrow size and hookline length scale with anno_scale; also
  corrects world_offset subtraction that was previously deferred to the
  TruckObject::Lines path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/multileader.rs

@@ -11,7 +11,7 @@ use crate::scene::wire_model::{SnapHint, TangentGeom};
 // ── TruckConvertible ────────────────────────────────────────────────────────
 
 /// Catmull-Rom spline tessellation through `ctrl` points, `segs_per_span` segments each.
-fn catmull_rom_pts(ctrl: &[[f32; 3]], segs_per_span: u32) -> Vec<[f32; 3]> {
+pub(crate) fn catmull_rom_pts(ctrl: &[[f32; 3]], segs_per_span: u32) -> Vec<[f32; 3]> {
     let n = ctrl.len();
     let mut out = Vec::new();
     for i in 0..n.saturating_sub(1) {