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/draw/attdef.rs

@@ -161,6 +161,7 @@ impl CadCommand for AttdefCommand {
         let d = 0.15_f32;
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "attdef_preview".into(),
             points: vec![

src/modules/draw/draw/mline.rs

@@ -137,6 +137,7 @@ impl CadCommand for MlineCommand {
         pts.push([pt.x, pt.y, pt.z]);
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "mline_preview".into(),
             points: pts,

src/modules/draw/draw/raster_image.rs

@@ -109,6 +109,7 @@ impl CadCommand for ImageCommand {
 
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "image_preview".into(),
             points: vec![p0, p1, p2, p3, p0],

src/modules/draw/draw/ray.rs

@@ -85,6 +85,7 @@ impl CadCommand for RayCommand {
         let far = base + dir * DISPLAY_EXTENT;
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "ray_preview".into(),
             points: vec![[base.x, base.y, base.z], [far.x, far.y, far.z]],
@@ -177,6 +178,7 @@ impl CadCommand for XLineCommand {
         let far_neg = base - dir * DISPLAY_EXTENT;
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "xline_preview".into(),
             points: vec![

src/modules/draw/draw/revcloud.rs

@@ -90,6 +90,7 @@ impl CadCommand for RevCloudCommand {
         ]);
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "revcloud_preview".into(),
             points: preview_pts,

src/modules/draw/draw/wipeout.rs

@@ -115,6 +115,7 @@ impl CadCommand for WipeoutCommand {
                 let max = p1.max(pt);
                 Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
                     name: "wipeout_preview".into(),
                     points: vec![
@@ -158,6 +159,7 @@ impl CadCommand for WipeoutCommand {
                 ]);
                 Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
                     name: "wipeout_poly_preview".into(),
                     points: pts,

src/modules/draw/inquiry/area.rs

@@ -70,6 +70,7 @@ impl CadCommand for AreaCommand {
         pts.push(f(self.points[0]));
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "area_preview".into(),
             points: pts,

src/modules/draw/inquiry/dist.rs

@@ -64,6 +64,7 @@ impl CadCommand for DistCommand {
         // The preview wire is purely visual, so f32 vertices are fine here.
         Some(WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: "dist_preview".into(),
             points: vec![

src/modules/draw/inquiry/measuregeom.rs

@@ -68,6 +68,7 @@ impl MeasureGeomCommand {
     fn preview_wire(name: &str, pts: Vec<[f32; 3]>) -> WireModel {
         WireModel {
             dash_from_start: false,
+            dash_align_end: None,
             text_verts: Vec::new(),
             name: name.to_string(),
             points: pts,

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,