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/app/mod.rs

@@ -205,6 +205,10 @@ pub(super) struct OpenCADStudio {
     /// until dismissed (click outside, ESC, cursor leaves the grip).
     grip_popup: Option<GripPopup>,
     grip_pending: Option<GripPendingValue>,
+    /// A leader line just added via the "Add Leader" grip menu whose arrow is
+    /// being placed (follows the cursor). `(entity handle, new-arrow grip id)`.
+    /// Esc before the placement click removes it again.
+    grip_add_provisional: Option<(acadrust::Handle, usize)>,
     /// Open Quick Select panel state. `None` = panel closed. Filters are
     /// applied via `Message::QSelectApply`; the panel is dismissed on
     /// Apply / Cancel / Esc / outside-click.
@@ -980,6 +984,7 @@ impl OpenCADStudio {
             grip_hover: None,
             grip_popup: None,
             grip_pending: None,
+            grip_add_provisional: None,
             qselect: None,
             show_ucs_icon: true,
             show_viewcube: true,