fix(precision): keep typed coordinates exact in f64 (#311)

Typed values were quantized to the f32 grid before reaching the
entity (e.g. a length of 2000.8 committed as 2000.8020), worsening
with magnitude. The interaction point layer was all glam::Vec3 (f32).

Two-layer fix:

1. Coordinate backbone -> f64. last_point, last_cursor_world,
   dyn_anchor, dyn_ref become DVec3; UcsXform transforms, parse_coord,
   the ucs helper fns, dyn_resolve_point, the OTRACK typed-distance
   paths, command-line coordinate entry and the coordinate readout all
   compute in f64. Screen/GPU/alignment stays f32 (snap engine, render
   geometry, projection, UCS-icon/viewcube drawing) with casts only at
   those boundaries.

2. Per-command committed storage -> DVec3. on_point already received a
   DVec3, but many command impls narrowed it back to f32 (let pt =
   pt.as_vec3(); Vec<Vec3> fields) and re-widened at build, quantizing
   the committed coordinate. Migrated PLINE, SPLINE, MLINE, LENGTHEN,
   INSERT, 3D primitives, every dimension/leader/mleader, ray, revcloud,
   wipeout, attdef, table, tolerance, array center, mview, plot_window,
   plus the QDIM_PLACE handler and qdim_collect_points. Preview,
   hit-test and rubber-band paths keep f32 at the boundary.

Hatch manual boundaries are left for a follow-up: HatchModel.boundary
is structurally Arc<Vec<[f32;2]>> and needs a model-level widen.

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

src/app/document.rs

@@ -169,7 +169,7 @@ pub(super) struct DocumentTab {
     pub(super) wireframe: bool,
     pub(super) render_mode: acadrust::entities::ViewportRenderMode,
     pub(super) visual_style: String,
-    pub(super) last_cursor_world: glam::Vec3,
+    pub(super) last_cursor_world: glam::DVec3,
     pub(super) last_cursor_screen: iced::Point,
     /// Base point (`App::last_point`) projected to viewport pixels, refreshed
     /// on cursor move. Lets the dynamic-input overlay place the distance label
@@ -185,10 +185,10 @@ pub(super) struct DocumentTab {
     pub(super) dyn_guide: crate::command::DynGuide,
     /// World-space anchor the current step's values are measured from. `None`
     /// falls back to `App::last_point`.
-    pub(super) dyn_anchor: Option<glam::Vec3>,
+    pub(super) dyn_anchor: Option<glam::DVec3>,
     /// Far end of a reference line through `dyn_anchor` (for the `Perp` guide:
     /// the base edge / major axis the offset is measured square to).
-    pub(super) dyn_ref: Option<glam::Vec3>,
+    pub(super) dyn_ref: Option<glam::DVec3>,
     /// `dyn_ref` projected to viewport pixels.
     pub(super) dyn_ref_screen: Option<iced::Point>,
     /// Index of the field that TAB has focused (the one keystrokes edit).
@@ -360,9 +360,9 @@ impl DocumentTab {
         let (o, x, y, z) = self.ucs_xform().axes();
         let origin = glam::Vec3::new(o.x as f32, o.y as f32, o.z as f32);
         glam::Mat4::from_cols(
-            x.extend(0.0),
-            y.extend(0.0),
-            z.extend(0.0),
+            x.as_vec3().extend(0.0),
+            y.as_vec3().extend(0.0),
+            z.as_vec3().extend(0.0),
             origin.extend(1.0),
         )
     }
@@ -421,7 +421,7 @@ impl DocumentTab {
             wireframe: false,
             render_mode: acadrust::entities::ViewportRenderMode::Wireframe2D,
             visual_style: "Wireframe 2D".into(),
-            last_cursor_world: glam::Vec3::ZERO,
+            last_cursor_world: glam::DVec3::ZERO,
             last_cursor_screen: iced::Point::ORIGIN,
             last_point_screen: None,
             dyn_fields: Vec::new(),