refactor(wire): add WireModel.points_low parallel buffer (empty default)

Foundation for double-single positions: WireModel now carries an
index-paired f32 residual buffer (`points_low`) next to the existing
`points` high half. Empty means "all-zero residual", so every existing
constructor (interactive draw, previews) is unchanged in behaviour;
tessellation from CAD f64 will fill it in the next steps so vertex
precision survives at UTM-scale coordinates.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/modules/annotate/angular_dim.rs

@@ -126,6 +126,7 @@ fn preview_wire(points: Vec<Vec3>) -> WireModel {
     WireModel {
         name: "dimangular_preview".to_string(),
         points: points.into_iter().map(|p| [p.x, p.y, p.z]).collect(),
+        points_low: Vec::new(),
         color: WireModel::CYAN,
         selected: false,
         pattern_length: 0.0,