feat(dynamic-input): per-step specs for rectangle, polygon and ellipse

Extend the DynSpec coverage and add the guides those commands need:

- Rectangle: two-corner enters width/height as unsigned magnitudes (the
  sign follows the cursor side, like the angle entry), drawn with the
  rectangle as the guide; rotated rectangle's height is a single distance
  measured square to the base edge (dimension line offset off the edge
  with extension lines); centre rectangle shows half-width/half-height.
- Polygon: inscribed/circumscribed vertex step shows radius + rotation.
- Ellipse: minor-axis step (all three modes) measures the half-length
  perpendicular to the major axis and draws that semi-axis from the
  centre; angle steps anchor their arc at the centre.

Adds DynGuide::Perp / PerpDim (perpendicular measure to a reference line)
and DynSpec::ref_point, plus Width/Height role handling (unsigned display,
cursor-signed commit) and diameter value scaling.

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

src/modules/home/draw/circle.rs

@@ -201,6 +201,7 @@ impl CadCommand for CircleCommand {
                 anchor: DynAnchor::Point(c),
                 fields: vec![DynFieldSpec::new(DynRole::Radius)],
                 guide: DynGuide::Radius,
+                ref_point: None,
             }),
         }
     }
@@ -288,6 +289,7 @@ impl CadCommand for CircleCDCommand {
                 anchor: DynAnchor::Point(c),
                 fields: vec![DynFieldSpec::new(DynRole::Diameter)],
                 guide: DynGuide::Radius,
+                ref_point: None,
             }),
         }
     }