fix(grips): compute grip positions in f64 so they sit on the wire

Grip markers drew visibly offset from the entity on large-coordinate
(UTM-scale) drawings: every Grippable impl cast the f64 entity coordinates
to f32 BEFORE the world-offset subtraction, and at 1e7 magnitude an f32
holds ~0.9 drawing units of rounding error. The wires subtract the offset
in f64 first (precision-preserving), so the grips landed up to ~1 unit
away from the geometry they belong to.

GripDef.world is now DVec3: producers fill it straight from the f64
entity data, refresh_selected_grips subtracts the world offset in f64,
and only the small offset-local result is cast to f32 for screen-space
projection (grips_to_screen / find_hit_grip) and drag state.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/grip.rs

@@ -40,7 +40,7 @@ pub fn grips_to_screen(
     grips
         .iter()
         .map(|g| {
-            let ndc = view_proj.project_point3(g.world);
+            let ndc = view_proj.project_point3(g.world.as_vec3());
             let screen = Point::new(
                 bounds.x + (ndc.x + 1.0) * 0.5 * bounds.width,
                 bounds.y + (1.0 - ndc.y) * 0.5 * bounds.height,
@@ -64,8 +64,8 @@ pub fn grips_to_screen_paper(
         .iter()
         .map(|g| {
             let screen = Point::new(
-                (g.world.x - tx + half_w) / (2.0 * half_w) * bounds.width,
-                (ty + half_h - g.world.y) / (2.0 * half_h) * bounds.height,
+                (g.world.x as f32 - tx + half_w) / (2.0 * half_w) * bounds.width,
+                (ty + half_h - g.world.y as f32) / (2.0 * half_h) * bounds.height,
             );
             (g.id, screen, g.is_midpoint, g.shape, g.dir)
         })
@@ -87,15 +87,15 @@ pub fn find_hit_grip_paper(
 
     for g in grips {
         let screen = Point::new(
-            (g.world.x - tx + half_w) / (2.0 * half_w) * bounds.width,
-            (ty + half_h - g.world.y) / (2.0 * half_h) * bounds.height,
+            (g.world.x as f32 - tx + half_w) / (2.0 * half_w) * bounds.width,
+            (ty + half_h - g.world.y as f32) / (2.0 * half_h) * bounds.height,
         );
         let dx = screen.x - cursor.x;
         let dy = screen.y - cursor.y;
         let d = (dx * dx + dy * dy).sqrt();
         if d < best_dist {
             best_dist = d;
-            best = Some((g.id, g.is_midpoint, g.world));
+            best = Some((g.id, g.is_midpoint, g.world.as_vec3()));
         }
     }
     best
@@ -113,7 +113,7 @@ pub fn find_hit_grip(
     let mut best: Option<(usize, bool, Vec3)> = None;
 
     for g in grips {
-        let ndc = view_proj.project_point3(g.world);
+        let ndc = view_proj.project_point3(g.world.as_vec3());
         let screen = Point::new(
             (ndc.x + 1.0) * 0.5 * bounds.width,
             (1.0 - ndc.y) * 0.5 * bounds.height,
@@ -123,7 +123,7 @@ pub fn find_hit_grip(
         let d = (dx * dx + dy * dy).sqrt();
         if d < best_dist {
             best_dist = d;
-            best = Some((g.id, g.is_midpoint, g.world));
+            best = Some((g.id, g.is_midpoint, g.world.as_vec3()));
         }
     }
     best

src/scene/object.rs

@@ -75,8 +75,12 @@ pub enum GripShape {
 pub struct GripDef {
     /// Object-local identifier (stable index, unique per object instance).
     pub id: usize,
-    /// World-space position of the grip.
-    pub world: Vec3,
+    /// World-space position of the grip, in f64. Entity coordinates can sit at
+    /// UTM magnitudes (1e7); casting to f32 before the world-offset subtraction
+    /// loses ~1 drawing unit and draws the grip visibly off the wire. Producers
+    /// fill this straight from the f64 entity data; the offset is subtracted in
+    /// f64 and only then cast for screen-space math.
+    pub world: glam::DVec3,
     /// `true` → midpoint / centre grip (drags the whole shape).
     /// `false` → endpoint / vertex grip (moves a single point).
     pub is_midpoint: bool,