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>
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