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/qdim.rs

@@ -90,6 +90,7 @@ impl CadCommand for QdimCommand {
         Some(WireModel {
             name: "qdim_preview".into(),
             points: vec![[pt.x - d * 3.0, pt.y, pt.z], [pt.x + d * 3.0, pt.y, pt.z]],
+            points_low: Vec::new(),
             color: WireModel::CYAN,
             selected: false,
             pattern_length: 0.0,