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/model/object.rs

@@ -1,7 +1,7 @@
 // Shared value types used by the dispatch and grip systems.
 
 use acadrust::types::{Color as AcadColor, LineWeight};
-use glam::Vec3;
+use glam::{DVec3, Vec3};
 
 /// The kind of value held by a property row.
 #[derive(Clone, Debug, PartialEq)]
@@ -96,9 +96,9 @@ pub struct GripDef {
 #[derive(Clone, Debug)]
 pub enum GripApply {
     /// Move a specific vertex to this absolute world position.
-    Absolute(Vec3),
+    Absolute(DVec3),
     /// Translate the whole object by this delta vector.
-    Translate(Vec3),
+    Translate(DVec3),
 }
 
 /// One entry in the hover-popup menu that opens when the cursor dwells