Dimensional constraints: dynamic dimension persistence, lock mark, display variables, DIMCONSTRAINT

The dynamic dimension of a dimensional constraint is now linked in the
drawing file (AcDbAssocDependency + AcDbAssocDimDependencyBody on the
constraint node), reread on load, hidden by DCHIDE/DYNCONSTRAINTDISPLAY
and never plotted; it carries a lock mark before its text. New
CONSTRAINTNAMEFORMAT and DYNCONSTRAINTDISPLAY variables and the
DIMCONSTRAINT option front end. A transformed entity whose own end points
a driving dimension measures keeps only the first constraint point, so a
scaled line returns to its dimension value.

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

src/entities/dimension.rs

@@ -6295,6 +6295,45 @@ fn dimension_text_is_outside(dim: &Dimension, style: Option<&DimStyle>) -> bool
         }
 }
 
+/// Where a dynamic dimension's lock mark sits: just before the text on its
+/// baseline, with the direction that points away from the text.
+pub(crate) fn dynamic_dimension_lock_anchor(
+    document: &CadDocument,
+    dim: &Dimension,
+    anno_scale: f64,
+) -> Option<(Vector3, Vector3)> {
+    let style_name = &dim.base().style_name;
+    let source_style = document.dim_styles.iter().find(|s| {
+        s.name.eq_ignore_ascii_case(style_name)
+            || (style_name.trim().is_empty() && s.name.eq_ignore_ascii_case("Standard"))
+    });
+    let effective_style = source_style.map(|style| resolved_dimension_style(style, dim, document));
+    let style = effective_style.as_ref();
+    let dim_scale = style
+        .map(|s| if s.dimscale > 1e-6 { s.dimscale } else { anno_scale })
+        .unwrap_or(1.0);
+    let text_height = style.map(|s| s.dimtxt * dim_scale).unwrap_or(2.5);
+    let value = dimension_text_value(dim, style)?;
+    let stack_scale = style.map(dimtfac_or_one).unwrap_or(1.0);
+    let half_width = text_cells(&value, stack_scale) * text_height * CELL_WIDTH * 0.5;
+    let pos = dimension_text_pos_f64(dim, style, text_height, dim_scale);
+    let rot = if dim.base().text_rotation.abs() > 1e-9 {
+        dim.base().text_rotation
+    } else {
+        dimension_text_natural_rotation(dim)
+    };
+    let (sr, cr) = rot.sin_cos();
+    let outward = Vector3::new(-cr, -sr, 0.0);
+    Some((
+        Vector3::new(
+            pos.x + outward.x * half_width,
+            pos.y + outward.y * half_width,
+            pos.z,
+        ),
+        outward,
+    ))
+}
+
 fn dimension_text_natural_rotation(dim: &Dimension) -> f64 {
     let angle = match dim {
         Dimension::Linear(d) => d.rotation,