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 <[email protected]>

src/app/update.rs

@@ -3055,6 +3055,7 @@ impl OpenCADStudio {
                                         [far_neg.x, far_neg.y, far_neg.z],
                                         [far_pos.x, far_pos.y, far_pos.z],
                                     ],
+                                    points_low: Vec::new(),
                                     color: [0.2, 0.7, 0.9, 0.6],
                                     selected: false,
                                     aci: 0,
@@ -3086,6 +3087,7 @@ impl OpenCADStudio {
                                     [far_neg.x, far_neg.y, far_neg.z],
                                     [far_pos.x, far_pos.y, far_pos.z],
                                 ],
+                                points_low: Vec::new(),
                                 color: [0.2, 0.9, 0.5, 0.6],
                                 selected: false,
                                 aci: 0,