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/leader.rs

@@ -589,6 +589,7 @@ impl LeaderTess for Leader {
 
         if verts.len() < 2 {
             return WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                 name,
                 points: vec![],
@@ -691,6 +692,7 @@ impl LeaderTess for Leader {
         }
 
         WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
             name,
             points,