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/scene/view/camera.rs

@@ -233,9 +233,24 @@ impl Camera {
     }
 
     pub fn pick_on_target_plane(&self, screen: Point, bounds: Rectangle) -> Vec3 {
+        self.pick_on_target_plane_f64(screen, bounds).as_vec3()
+    }
+
+    /// f64 variant of [`pick_on_plane`] — keeps the cursor precise at UTM-scale.
+    pub fn pick_on_plane_f64(
+        &self,
+        screen: Point,
+        bounds: Rectangle,
+        plane_normal: Vec3,
+        plane_point: glam::DVec3,
+    ) -> glam::DVec3 {
+        self.unproject_on_plane_f64(screen, bounds, plane_normal, plane_point)
+    }
+
+    /// f64 variant of [`pick_on_target_plane`].
+    pub fn pick_on_target_plane_f64(&self, screen: Point, bounds: Rectangle) -> glam::DVec3 {
         let forward = (self.target.as_vec3() - self.eye()).normalize_or(Vec3::NEG_Z);
         self.unproject_on_plane_f64(screen, bounds, forward, self.target)
-            .as_vec3()
     }