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/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,

src/scene/convert/tessellate.rs

@@ -169,6 +169,7 @@ pub fn tessellate(
                     return vec![WireModel {
                         name,
                         points: Vec::new(),
+                        points_low: Vec::new(),
                         color,
                         selected,
                         pattern_length: 0.0,
@@ -207,6 +208,7 @@ pub fn tessellate(
                     out.push(WireModel {
                         name: name.clone(),
                         points: pts,
+                        points_low: Vec::new(),
                         color: wire_color,
                         selected,
                         pattern_length: 0.0,
@@ -248,6 +250,7 @@ pub fn tessellate(
                                 [x, y - s, z],
                                 [x, y + s, z],
                             ],
+                            points_low: Vec::new(),
                             color,
                             selected,
                             pattern_length: 0.0,
@@ -279,6 +282,7 @@ pub fn tessellate(
                     return vec![WireModel {
                         name,
                         points,
+                        points_low: Vec::new(),
                         color,
                         selected,
                         pattern_length,
@@ -308,6 +312,7 @@ pub fn tessellate(
                     return vec![WireModel {
                         name,
                         points,
+                        points_low: Vec::new(),
                         color,
                         selected,
                         pattern_length,
@@ -355,6 +360,7 @@ pub fn tessellate(
                 return vec![WireModel {
                     name,
                     points: local_pts,
+                    points_low: Vec::new(),
                     color,
                     selected,
                     pattern_length: 0.0,
@@ -392,6 +398,7 @@ pub fn tessellate(
                 return vec![WireModel {
                     name,
                     points: local_pts,
+                    points_low: Vec::new(),
                     color,
                     selected,
                     pattern_length,
@@ -432,6 +439,7 @@ pub fn tessellate(
     vec![WireModel {
         name,
         points,
+        points_low: Vec::new(),
         color,
         selected,
         aci: 0,

src/scene/mod.rs

@@ -8196,6 +8196,7 @@ fn tessellate_entity(
         let marker = WireModel {
             name: h.value().to_string(),
             points: vec![],
+            points_low: Vec::new(),
             color: entity_color,
             selected: sel,
             aci: 0,
@@ -8407,6 +8408,7 @@ fn tessellate_entity(
                 return vec![WireModel {
                     name: h.value().to_string(),
                     points: pts,
+                    points_low: Vec::new(),
                     color: entity_color,
                     selected: sel,
                     aci,
@@ -8444,6 +8446,7 @@ fn tessellate_entity(
                 return vec![WireModel {
                     name: h.value().to_string(),
                     points: vec![],
+                    points_low: Vec::new(),
                     color: fill_color,
                     selected: sel,
                     aci,
@@ -8552,6 +8555,7 @@ fn lod_stub_wire(
         // different elevation. 2D entities (text fallbacks) pass
         // z_min = z_max = 0 to keep the historical behaviour.
         points: vec![[ax, ay, z_min], [bx, by, z_max]],
+        points_low: Vec::new(),
         color: stored_color,
         selected,
         aci,
@@ -8607,6 +8611,7 @@ fn lod_stub_wire_3d(
     WireModel {
         name,
         points,
+        points_low: Vec::new(),
         color: stored_color,
         selected,
         aci,

src/scene/model/wire_model.rs

@@ -34,8 +34,14 @@ pub enum TangentGeom {
 pub struct WireModel {
     /// Unique identifier — the handle value as a decimal string.
     pub name: String,
-    /// Ordered world-space positions forming a strip of quads.
+    /// Ordered world-space positions forming a strip of quads. Each entry is
+    /// the "high" half of a double-single f32 pair; [`points_low`] carries the
+    /// matching residual so the shader can reconstruct the f64 source.
     pub points: Vec<[f32; 3]>,
+    /// Low-bit residual paired index-for-index with [`points`]. Empty means
+    /// "all-zero residual" (interactive draw / preview wires whose coordinates
+    /// don't need sub-f32 precision). Tessellation from CAD f64 fills it.
+    pub points_low: Vec<[f32; 3]>,
     /// RGBA colour in [0, 1].
     pub color: [f32; 4],
     /// Whether this wire is currently selected.
@@ -94,6 +100,7 @@ impl WireModel {
         Self {
             name,
             points,
+            points_low: Vec::new(),
             color,
             selected,
             aci: 0,
@@ -216,6 +223,7 @@ impl Default for WireModel {
         Self {
             name: String::new(),
             points: Vec::new(),
+            points_low: Vec::new(),
             color: Self::WHITE,
             selected: false,
             pattern_length: 0.0,

src/scene/text/complex_lt.rs

@@ -210,6 +210,7 @@ pub fn apply_along(
         .map(|pts| WireModel {
             name: name.to_string(),
             points: pts,
+            points_low: Vec::new(),
             color,
             selected,
             pattern_length: 0.0,