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/command_driver.rs

@@ -137,7 +137,7 @@ impl OpenCADStudio {
             };
             self.last_point = Some(wcs);
             self.push_ucs_to_cmd(i);
-            let _ = self.feed_command(StepInput::Point(wcs.as_dvec3()));
+            let _ = self.feed_command(StepInput::Point(wcs));
         } else {
             let _ = self.feed_command(StepInput::Text(token.to_string()));
         }
@@ -1156,7 +1156,7 @@ impl OpenCADStudio {
                     .scene
                     .document
                     .get_entity(handle)
-                    .and_then(|e| break_entity(e, p1.as_vec3(), p2.as_vec3()));
+                    .and_then(|e| break_entity(e, p1, p2));
                 match replacement {
                     Some(frags) => {
                         let label = self.history_label_from_active_cmd(i, "BREAK");