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 <noreply@anthropic.com>

src/scene/convert/tess.rs

@@ -482,6 +482,7 @@ pub(crate) fn tessellate_entity(
         );
         let marker = WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: h.value().to_string(),
             points: vec![],
@@ -693,6 +694,9 @@ pub(crate) fn tessellate_entity(
                 entity_color,
                 sel,
                 base.line_weight_px,
+                // Single (non-MLINE) entity: keep the from-start tiling (no shared
+                // A-type reference — there are no sibling elements to align with).
+                None,
             );
             if !wires.is_empty() {
                 for w in &mut wires {
@@ -856,6 +860,7 @@ fn lod_stub_wire(
     let stored_color = if selected { WireModel::SELECTED } else { color };
     WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
         name,
         // Diagonal of the entity's 3D AABB so depth tests against
@@ -920,6 +925,7 @@ fn lod_stub_wire_3d(
     let stored_color = if selected { WireModel::SELECTED } else { color };
     WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
         name,
         points,