Dynamic dimensions: constant screen size, transform keeps the second constraint point

Every dynamic dimension gets a DIMSCALE override for 12 px text, refreshed
per zoom band, at creation and after load. A SCALE/ROTATE of an entity whose
own end points a driving dimension measures now pins its second constraint
point so the dimension moves the first end, as the reference does.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/scene/dwg_native_constraints.rs

@@ -2810,6 +2810,9 @@ impl Scene {
 
     pub(crate) fn load_parametric_constraints_from_document(&mut self) {
         self.parametric_constraints.clear();
+        // Dynamic dimensions read from the file get their screen size on the
+        // first tick after the view is known.
+        self.dynamic_dimension_band = 0.0;
         let owners: Vec<_> = self
             .document
             .block_records