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/scene/model/wire_model.rs

@@ -79,6 +79,13 @@ pub struct WireModel {
     /// from the START vertex with continuous phase and no A-type end forcing —
     /// DGN line styles are not end-aligned.
     pub dash_from_start: bool,
+    /// Shared "A"-type end-dash length for MLINE elements. `Some(len)` makes the
+    /// dash shader use `len` as the begin/end solid-dash length for EVERY
+    /// parallel element (derived once from the multiline's centre-line length),
+    /// while `align_total` stays each element's own length — so all elements
+    /// share one interior phase (perpendicular dashes line up) yet each still
+    /// ends on a dash. `None` (the default) = per-wire A-type / from-start.
+    pub dash_align_end: Option<f32>,
     /// Paper-space bounding box [x0, y0, x1, y1] for GPU scissor clipping.
     /// Set only for viewport-projected wires in paper-space layouts.
     pub vp_scissor: Option<[f32; 4]>,
@@ -158,6 +165,7 @@ impl WireModel {
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
             dash_from_start: false,
+            dash_align_end: None,
             vp_scissor: None,
             fill_tris: vec![],
             fill_tris_low: Vec::new(),
@@ -367,6 +375,7 @@ impl Default for WireModel {
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
             dash_from_start: false,
+            dash_align_end: None,
             vp_scissor: None,
             fill_tris: Vec::new(),
             fill_tris_low: Vec::new(),