feat(dynamic-input): ARC specs + ellipse-arc preview fixes

- ARC: per-step DynSpec for the typed-value modes — included-angle (SCA,
  CSA), chord length (SCL, CSL) and radius (SER) show their box and route
  the typed value through on_text_input via a new dyn_commit_as_text()
  hook; the angle box previews the live span. Point/3P/SCE steps stay on
  the legacy polar path.
- ARC + ellipse-arc: derive the previewed sweep direction from the swept
  angle with a ~10° tolerance and advance the reference only on a clear
  move, so small cursor jitter no longer flips the arc. Ellipse uses the
  parametric angle so a flat ellipse isn't sticky.
- Ellipse arc: the start-angle step now shows a line from the centre to
  the cursor (it only picks the start angle) instead of a misleading arc.
- Ellipse arc: use the same minor-axis direction (Z x major) as the
  rendered entity so the committed arc matches the preview instead of
  mirroring across the major axis.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/command/mod.rs

@@ -567,6 +567,15 @@ pub trait CadCommand: Send {
         DynField::Point
     }
 
+    /// When true, a value typed into the dynamic-input box for this step is
+    /// committed via `on_text_input` (as a string the command parses) rather
+    /// than resolved into a point. Used by steps whose typed value is a span /
+    /// included angle / length the command interprets itself (e.g. ARC angle
+    /// modes), while the box still previews a live value from the cursor.
+    fn dyn_commit_as_text(&self) -> bool {
+        false
+    }
+
     /// Explicit per-step dynamic-input description. `Some(spec)` takes full
     /// control of the boxes, guide geometry and anchor for this step; `None`
     /// (the default) falls back to the legacy `dyn_field()` behaviour so