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/convert/dgn_linestyle.rs

@@ -201,6 +201,15 @@ pub fn offset_host_entity(e: &EntityType, d: f64) -> Option<EntityType> {
     let mut clone = e.clone();
     match &mut clone {
         EntityType::LwPolyline(p) => {
+            // Drop consecutive duplicate vertices first. A zero-length segment
+            // gives `offset_xy` a degenerate normal, which leaves that vertex
+            // un-offset — folding the wall down to the centre line and drawing a
+            // spurious segment that visually links the two walls. (Some DGN pipe
+            // polylines carry trailing duplicate end vertices.)
+            p.vertices.dedup_by(|a, b| {
+                (a.location.x - b.location.x).abs() < 1e-9
+                    && (a.location.y - b.location.y).abs() < 1e-9
+            });
             if p.vertices.len() < 2 {
                 return None;
             }
@@ -215,6 +224,10 @@ pub fn offset_host_entity(e: &EntityType, d: f64) -> Option<EntityType> {
             }
         }
         EntityType::Polyline2D(p) => {
+            p.vertices.dedup_by(|a, b| {
+                (a.location.x - b.location.x).abs() < 1e-9
+                    && (a.location.y - b.location.y).abs() < 1e-9
+            });
             if p.vertices.len() < 2 {
                 return None;
             }