DCDIAMETER/DCRADIUS: the reference flow with a real radial dimension

The commands ask for the circle or arc themselves, report the measured
size, take a dimension line location and then the value or name and
value, creating a radius or diameter dimension as the constraint's
dynamic dimension with a radN/diaN parameter, refusing a second radial
constraint on the same object and any non-positive value.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/command.rs

@@ -1635,6 +1635,22 @@ pub enum CmdResult {
         renamed: bool,
         label: &'static str,
     },
+    /// Adds a radius or diameter constraint with its dynamic radial
+    /// dimension and the `radN`/`diaN` parameter that drives it.
+    AddRadialConstraint {
+        circle: crate::scene::parametric_constraints::ParametricRef,
+        center: DVec3,
+        radius: f64,
+        /// Where the dimension line was picked; its direction from the
+        /// centre places the dimension.
+        location: DVec3,
+        diameter: bool,
+        name: String,
+        expression: String,
+        /// The user named the parameter (`name=expression`); an existing
+        /// name is then refused.
+        renamed: bool,
+    },
     /// Adds an angular constraint with its dynamic angular dimension and
     /// the `angN` parameter that drives it. Two lines: `refs` =
     /// `[first_line, second_line]`, `points` = both lines' ends; three