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/table_cmd.rs

@@ -150,6 +150,7 @@ impl CadCommand for TableCommand {
                     [x, y, z - h],
                     [x, y, z],
                 ],
+                points_low: Vec::new(),
                 color: WireModel::CYAN,
                 selected: false,
                 pattern_length: 0.0,