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/app/view.rs

@@ -763,6 +763,7 @@ impl OpenCADStudio {
                 Some(overlay::dynamic_input_overlay(
                     tab.last_cursor_screen,
                     tab.last_point_screen,
+                    tab.dyn_ref_screen,
                     tab.dyn_guide,
                     boxes,
                     prompt,
@@ -4084,9 +4085,12 @@ fn dyn_component_value(
     // in `dyn_resolve_point` so the live preview and the committed
     // coordinate use the same frame. See #35.
     let has_base = base.is_some();
+    // Width / Height read as unsigned magnitudes (the sign is taken from the
+    // cursor side on commit), matching the rectangle's two-edge entry.
+    let wh = matches!(f.role, crate::command::DynRole::Width | crate::command::DynRole::Height);
     match f.component {
-        DynComponent::X if has_base => format!("{:.4}", dx),
-        DynComponent::Y if has_base => format!("{:.4}", dy),
+        DynComponent::X if has_base => format!("{:.4}", if wh { dx.abs() } else { dx }),
+        DynComponent::Y if has_base => format!("{:.4}", if wh { dy.abs() } else { dy }),
         DynComponent::Z if has_base => "0.0000".to_string(),
         DynComponent::X => format!("{:.4}", w.x),
         DynComponent::Y => format!("{:.4}", w.y),