wip(f64): convert command layer to DVec3 (in progress)

Migrate the command/snap/grip/cursor chain from f32 Vec3 to f64 DVec3 so
picked points, snaps and grip drags stay precise at UTM-scale coordinates
(f32 quantizes to ~0.5 m there).

Done so far:
- command/mod.rs: EntityTransform, CmdResult points, CadCommand point
  methods, TangentObject already DVec3 (radius f64).
- snap: SnapResult.world DVec3; world_to_screen/try_pt DVec3.
- GripApply -> DVec3; grip find/state (GripEdit) f64; camera
  pick_on_plane_f64 / pick_on_target_plane_f64; scene paper_to_model_f64.
- update.rs click + mouse-move + grip-drag pipelines route precise DVec3
  to on_point/on_preview_wires/on_entity_pick; ortho/polar cast at the
  UcsXform boundary only.
- cmd_result dispatch, transform helpers, dimension/arc/circle/multileader
  apply_grip, circle/arc/ellipse/shapes draw commands, array modify.

Tree does NOT compile yet (~236 errors remain across modify/annotate/
insert command files + entity apply_grip impls); committed at user request
to checkpoint progress.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/entities/circle.rs

@@ -166,11 +166,9 @@ fn apply_grip(circle: &mut Circle, grip_id: usize, apply: GripApply) {
             circle.center.z += d.z as f64;
         }
         (1..=4, GripApply::Absolute(p)) => {
-            let cx = circle.center.x as f32;
-            let cy = circle.center.y as f32;
-            let dx = p.x - cx;
-            let dy = p.y - cy;
-            circle.radius = ((dx * dx + dy * dy) as f64).sqrt();
+            let dx = p.x - circle.center.x;
+            let dy = p.y - circle.center.y;
+            circle.radius = (dx * dx + dy * dy).sqrt();
         }
         _ => {}
     }