feat(dgn-ls): dash walls from start + fix wall connector and cap identity

DET pipe walls now carry the stroke pattern (dash_from_start), which
tiles from each wall's own start vertex and skips the A-type end
alignment that was force-lighting a solid dash under the end symbol.

Two rendering fixes on top:
- Dedup the host polyline's vertices before offsetting the walls. Some
  DGN pipe polylines carry trailing duplicate end vertices; a zero-length
  segment gave offset_xy a degenerate normal, leaving that vertex
  un-offset and folding the wall down to the centre line. The two walls'
  fold segments met at the centre and drew a spurious line linking them.
- Re-key the symbol (end circle / half-ellipse) wires to the host handle
  so the whole pipe picks and selects as one entity instead of the
  symbols acting as separate phantom entities.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/modules/draw/inquiry/area.rs

@@ -69,6 +69,7 @@ impl CadCommand for AreaCommand {
         pts.push(f(pt));
         pts.push(f(self.points[0]));
         Some(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
             name: "area_preview".into(),
             points: pts,

src/modules/draw/inquiry/dist.rs

@@ -63,6 +63,7 @@ impl CadCommand for DistCommand {
         let p1 = self.first?;
         // The preview wire is purely visual, so f32 vertices are fine here.
         Some(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
             name: "dist_preview".into(),
             points: vec![

src/modules/draw/inquiry/measuregeom.rs

@@ -67,6 +67,7 @@ impl MeasureGeomCommand {
     /// Build a cyan preview wire connecting the picked points and the cursor.
     fn preview_wire(name: &str, pts: Vec<[f32; 3]>) -> WireModel {
         WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
             name: name.to_string(),
             points: pts,