feat(mline): GPU-dash multiline elements with shared A-type alignment

MLINE dashed elements were fully CPU-expanded (apply_along) — one wire per
dash — to keep the parallel lines in phase. Move pure-dash elements onto the
GPU dash shader and end-align every element to the multiline's centre-line so
they share one interior phase.

- wire_distances now accumulates arc-length from the double-single (high+low)
  points, so the dash phase stays precise at UTM coordinates. f32-high-only
  deltas quantised ~0.1 there and drifted the parallel lines apart — the
  original reason MLINE dashes were CPU-side.
- New WireModel.dash_align_end carries a shared begin/end dash length. The
  shader uses it as align_end for every parallel element (the interior phase
  depends on align_end, not the wire's own length) while align_total stays the
  element's own length, so each still ends on a dash at its own endpoint.
- MLINE tessellation derives that length once from the centre-line, routes
  pure dash/space/dot elements to the GPU pattern and text-bearing elements to
  apply_along.
- apply_along gained an "A"-type mode (shared reference length): a solid
  begin/end dash and a phased interior, so its dashes — and embedded text —
  line up with the GPU-dashed sibling elements instead of tiling from start.

Pure-dash MLINEs now cost one WireModel + pattern per element instead of N
segments; single (non-MLINE) entities keep the from-start tiling unchanged.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/modules/draw/modify/trim.rs

@@ -3060,6 +3060,7 @@ fn collect_runs(pts: &[[f64; 2]], take: &dyn Fn([f64; 2]) -> bool, out: &mut Vec
 fn preview_wire(points: Vec<[f32; 3]>, color: [f32; 4], name: &str) -> WireModel {
     WireModel {
         dash_from_start: false,
+        dash_align_end: None,
         text_verts: Vec::new(),
         name: name.into(),
         points,