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/entities/table.rs

@@ -702,6 +702,7 @@ pub fn tessellate_table(
             WireModel {
                 name: name.clone(),
                 points,
+                points_low: Vec::new(),
                 color,
                 selected,
                 pattern_length: 0.0,