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 <[email protected]>

src/modules/view/plot_window.rs

@@ -5,10 +5,10 @@
 
 use crate::command::{CadCommand, CmdResult};
 use crate::scene::model::wire_model::WireModel;
-use glam::{DVec3, Vec3};
+use glam::DVec3;
 
 pub struct PlotWindowCommand {
-    p1: Option<Vec3>,
+    p1: Option<DVec3>,
 }
 
 impl PlotWindowCommand {
@@ -30,13 +30,13 @@ impl CadCommand for PlotWindowCommand {
         }
     }
 
-    fn on_point(&mut self, pt: DVec3) -> CmdResult { let pt = pt.as_vec3();
+    fn on_point(&mut self, pt: DVec3) -> CmdResult {
         match self.p1 {
             None => {
                 self.p1 = Some(pt);
                 CmdResult::NeedPoint
             }
-            Some(p1) => CmdResult::SetPlotWindow { p1: p1.as_dvec3(), p2: pt.as_dvec3() },
+            Some(p1) => CmdResult::SetPlotWindow { p1, p2: pt },
         }
     }
 
@@ -45,7 +45,7 @@ impl CadCommand for PlotWindowCommand {
     }
 
     fn on_mouse_move(&mut self, pt: DVec3) -> Option<WireModel> { let pt = pt.as_vec3();
-        let p1 = self.p1?;
+        let p1 = self.p1?.as_vec3();
         // Draw the selection rectangle.
         Some(WireModel {
             text_verts: Vec::new(),