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/entities/multileader.rs

@@ -1399,6 +1399,7 @@ impl MultiLeaderTess for MultiLeader {
         // WireModels so the renderer respects per-piece coloring.
         let mut wires: Vec<WireModel> = Vec::new();
         wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
             name: name.clone(),
             points,
@@ -1682,6 +1683,7 @@ impl MultiLeaderTess for MultiLeader {
                         xy = xy.max(y);
                     }
                     wires.push(WireModel {
+                        dash_from_start: false,
                         text_verts: sdf_verts,
                         name: name.clone(),
                         points: vec![],
@@ -1745,6 +1747,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[3],
                     ];
                     wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                         name: name.clone(),
                         points: vec![],
@@ -1776,6 +1779,7 @@ impl MultiLeaderTess for MultiLeader {
                         wcs_corners[0],
                     ];
                     wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                         name,
                         points: frame_points,