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/entities/leader.rs

@@ -44,7 +44,8 @@ fn to_truck(leader: &Leader) -> TruckEntity {
         let dy = next.y - tip.y;
         let len = (dx * dx + dy * dy).sqrt().max(1e-9);
         let (dx, dy) = (dx / len, dy / len);
-        let sz = (leader.text_height).max(1.0) * 0.8;
+        // Arrowhead sized to the text height, matching the MLEADER arrowhead.
+        let sz = (leader.text_height).max(1.0);
         let a = std::f64::consts::PI / 6.0;
         let (s, c) = a.sin_cos();
         let tip_f = p3(tip);
@@ -627,7 +628,7 @@ impl LeaderTess for Leader {
                 .unwrap_or(anno_scale as f64);
             let arrow_size = match style {
                 Some(s) => (s.dimasz * dim_scale) as f32,
-                None => (self.text_height as f32).max(1.0) * 0.8 * anno_scale,
+                None => (self.text_height as f32).max(1.0) * anno_scale,
             };
             let arrow = match style {
                 Some(s) => arrow_from_block(document, s.dimldrblk, arrow_size.max(0.001)),