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/app/update/viewport.rs

@@ -1236,6 +1236,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                                 let far_neg = base - dir * far;
                                 let guide = crate::scene::WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
                                     name: "__polar_guide__".into(),
                                     points: vec![
@@ -1271,6 +1272,7 @@ pub(super) fn on_tick(&mut self, t: Instant) -> Task<Message> {
                             let far_neg = h.base - h.dir * far;
                             previews.push(crate::scene::WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
                                 name: "__otrack_guide__".into(),
                                 points: vec![