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/scene/cache/block_cache.rs

@@ -1106,6 +1106,7 @@ impl Batches {
                 WireModel {
                     name: name.to_string(),
                     points: b.points,
+                    points_low: Vec::new(),
                     color,
                     selected,
                     pattern_length: b.pattern_length,