fix: review fixes for the metric template and the radial constraint

A new drawing (#1441) recorded ISO-25 as the current dimension style by
name only. Every other place that changes it sets the header's handle as
well, and a DWG carries the current style by handle alone, so a saved new
drawing reopened on Standard. The style's handle now comes back from the
helper that adds it.

A radius constraint (#1443) anchored the circle's centre from the shape of
its references — one whole entity — but a single whole reference is also a
line's length, whose start is what stays. The entity decides now.

The two tests that encoded the replaced rules follow them: a negative
radius validates by its magnitude, as a negative distance already did, and
only zero is refused; a new drawing's dimension text reads at ISO-25's
comma separator.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/scene/entity.rs

@@ -2733,8 +2733,11 @@ impl Scene {
         // current. Standard stays available for imperial work.
         self.document.header.measurement = 1;
         self.document.header.insertion_units = 4;
-        crate::scene::creation_style::ensure_iso_dim_style(&mut self.document);
+        let iso = crate::scene::creation_style::ensure_iso_dim_style(&mut self.document);
         self.document.header.current_dimstyle_name = "ISO-25".to_string();
+        // The name alone leaves the header pointing at the previous style, and
+        // a DWG writes the current style by handle only.
+        self.document.header.current_dimstyle_handle = iso;
         for obj in self.document.objects.values_mut() {
             if let acadrust::objects::ObjectType::Layout(l) = obj {
                 if l.name != "Model" {