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/dimension.rs

@@ -1352,6 +1352,7 @@ fn tessellate_dimension_inner(
             let (ext1, ext2) = split_ext_lines(&geom.ext_lines);
             if !ext1.is_empty() {
                 wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                     name: name.clone(),
                     points: ext1,
@@ -1374,6 +1375,7 @@ fn tessellate_dimension_inner(
             }
             if !ext2.is_empty() {
                 wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                     name: name.clone(),
                     points: ext2,
@@ -1396,6 +1398,7 @@ fn tessellate_dimension_inner(
             }
         } else {
             wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                 name: name.clone(),
                 points: geom.ext_lines,
@@ -1419,6 +1422,7 @@ fn tessellate_dimension_inner(
     }
 
     wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
         name: name.clone(),
         points: geom.dim_lines,
@@ -1458,6 +1462,7 @@ fn tessellate_dimension_inner(
                     aci_to_rgba(&c)
                 };
                 wires.push(WireModel {
+            dash_from_start: false,
             text_verts: Vec::new(),
                     name: name.clone(),
                     points: vec![],