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/scene/model/wire_model.rs

@@ -73,6 +73,12 @@ pub struct WireModel {
     /// When false the linetype pattern restarts at each NaN-separated segment
     /// (DXF PLINEGEN=0).  When true the pattern runs continuously (PLINEGEN=1).
     pub plinegen: bool,
+    /// DGN line-style marker. When false (every standard linetype) the dash
+    /// pattern uses the normal phase: A-type end alignment for dash-first
+    /// patterns, else centred. When true (DGN pipe walls) the pattern is drawn
+    /// from the START vertex with continuous phase and no A-type end forcing —
+    /// DGN line styles are not end-aligned.
+    pub dash_from_start: bool,
     /// Paper-space bounding box [x0, y0, x1, y1] for GPU scissor clipping.
     /// Set only for viewport-projected wires in paper-space layouts.
     pub vp_scissor: Option<[f32; 4]>,
@@ -151,6 +157,7 @@ impl WireModel {
             key_vertices: vec![],
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
+            dash_from_start: false,
             vp_scissor: None,
             fill_tris: vec![],
             fill_tris_low: Vec::new(),
@@ -359,6 +366,7 @@ impl Default for WireModel {
             key_vertices: Vec::new(),
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
+            dash_from_start: false,
             vp_scissor: None,
             fill_tris: Vec::new(),
             fill_tris_low: Vec::new(),