feat(leader): dynamic MLEADER text placement, grips, and consistent arrows

Reworks MLEADER text/leader geometry and grip editing, and aligns LEADER
to MLEADER's conventions (kept as separate entities).

Text & landing (render path):
- Text is side-anchored on the leader-facing edge and reads outward; the
  side, alignment and the horizontal landing are recomputed every frame
  from the text grip's position relative to the leader, so moving the
  arrow or the text re-mirrors the whole layout and the landing stays
  attached to the text's near edge.
- Fixes #49: text was centred on the leader endpoint regardless of side.

Grips:
- Text-location grip plus an MTEXT-style triangle wrap-width grip at the
  box's far edge; dragging it sets the MText wrap width.
- Multifunction grip menu restored and made functional: leader-line
  vertices offer Add Leader / Remove Leader (Add grabs the new arrow so it
  follows the cursor — click places, Esc removes it); the text grip offers
  Move Independent (text only) and Move with Leader (translates the whole
  multileader via a move-all sentinel grip).

Arrows:
- LEADER and MLEADER arrowheads now render at the same size (text height),
  matching previews.

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

src/modules/annotate/leader_cmd.rs

@@ -1,6 +1,6 @@
 // LEADER command
 //
-// Flow (matching the classic AutoCAD LEADER):
+// Flow:
 //   1. Click the arrowhead point, then one or more bend points.
 //   2. Enter (≥2 points) places the leader line plus a linked, empty MText
 //      annotation at the landing, and opens the in-place MText editor so the
@@ -132,7 +132,7 @@ fn build_mtext(text: &str, pos: Vec3, height: f64, attach: AttachmentPoint) -> M
 fn preview_wire(pts: &[Vec3]) -> WireModel {
     let mut points: Vec<[f32; 3]> = pts.iter().map(|p| [p.x, p.y, p.z]).collect();
     if pts.len() >= 2 {
-        let [w1, w2] = arrowhead_wings(pts[0], pts[1], 2.0);
+        let [w1, w2] = arrowhead_wings(pts[0], pts[1], 2.5);
         points.push([f32::NAN; 3]);
         points.push([w1.x, w1.y, w1.z]);
         points.push([pts[0].x, pts[0].y, pts[0].z]);