Aligned constraint: axis lines, zero collapse, Properties form/reference/description, screen-sized text

Point & line and 2Lines take a text baseline or an ellipse axis as the
line; a zero value folds the points onto the first one; 2Lines turns the
second line about its start. The Properties Constraint section edits
Constraint Form (Dynamic on the *ADSK_CONSTRAINTS layer, Annotational on
the current layer), Reference (driven constraint in parentheses whose
parameter follows the measurement) and Description (stored with the
parameter and the native variable). Dynamic dimensions keep a 12 px text
and 8 px arrows at any zoom, drop trailing zeros and show formulas as
`fx: len=d1*2`; the hidden constraints layer stays out of the layer lists.

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

src/app/layers.rs

@@ -62,6 +62,8 @@ impl OpenCADStudio {
             .layers
             .layers
             .iter()
+            // The reference's hidden system layers (`*ADSK_CONSTRAINTS`) stay out.
+            .filter(|l| !l.name.starts_with('*'))
             .map(|l| crate::ui::ribbon::LayerInfo {
                 name: l.name.clone(),
                 color: crate::ui::window::layers::iced_color_from_acad(&l.color),