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/mod.rs

@@ -3594,6 +3594,11 @@ impl Scene {
     /// geometry of the currently active viewport.  Returns the input unchanged
     /// when there is no active viewport.
     pub fn paper_to_model(&self, paper_pt: glam::Vec3) -> glam::Vec3 {
+        self.paper_to_model_f64(paper_pt.as_dvec3()).as_vec3()
+    }
+
+    /// f64 variant of [`paper_to_model`] — preserves UTM-scale precision.
+    pub fn paper_to_model_f64(&self, paper_pt: glam::DVec3) -> glam::DVec3 {
         let vp_handle = match self.active_viewport {
             Some(h) => h,
             None => return paper_pt,
@@ -3602,16 +3607,15 @@ impl Scene {
             Some(acadrust::EntityType::Viewport(vp)) => vp,
             _ => return paper_pt,
         };
-        let scale =
-            vp_effective_scale(vp.custom_scale, vp.view_height, vp.height) as f32;
+        let scale = vp_effective_scale(vp.custom_scale, vp.view_height, vp.height);
         if scale.abs() < 1e-9 {
             return paper_pt;
         }
-        let tx = vp.view_target.x as f32;
-        let ty = vp.view_target.y as f32;
-        let pcx = vp.center.x as f32;
-        let pcy = vp.center.y as f32;
-        glam::Vec3::new(
+        let tx = vp.view_target.x;
+        let ty = vp.view_target.y;
+        let pcx = vp.center.x;
+        let pcy = vp.center.y;
+        glam::DVec3::new(
             (paper_pt.x - pcx) / scale + tx,
             (paper_pt.y - pcy) / scale + ty,
             paper_pt.z,