DCRADIUS: reference radial dimensional constraint with a dynamic radial dimension

Select arc or circle / Dimension text / dimension line location / value or
name prompts with a radN parameter, invalid-selection, miss, duplicate and
invalid-expression messages, negative values as magnitudes, a dynamic radial
dimension on the constraints layer drawn as the reference does (mid-radius
text on the center-to-rim line, no hook, no center mark, one text grip), the
stored text position following the radius, center-anchored resizing, the
Properties title and Constraint section, DIMCONSTRAINT's Radius default,
the small-button Dimensional ribbon grid and tooltip descriptions.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/scene/parametric_solve.rs

@@ -996,14 +996,15 @@ fn resolved_target(params: &ParameterTable, constraint: &ParametricConstraint) -
     // A dimensional distance keeps its sign in the parameter but measures
     // its magnitude, as the reference does (`d1=-50` shortens the line to
     // 50; `0` folds the points together).
-    if constraint.kind == ConstraintKind::Distance {
-        return Some(value.abs());
+    // A radius or diameter likewise (`rad1=-20` is a radius of 20); zero is
+    // no circle at all.
+    match constraint.kind {
+        ConstraintKind::Distance => Some(value.abs()),
+        ConstraintKind::Radius | ConstraintKind::Diameter => {
+            (value != 0.0).then(|| value.abs())
+        }
+        _ => Some(value),
     }
-    let must_be_positive = matches!(
-        constraint.kind,
-        ConstraintKind::Radius | ConstraintKind::Diameter
-    );
-    (!must_be_positive || value > 0.0).then_some(value)
 }
 
 fn oriented_distance(target: f64, current_projection: f64) -> f64 {