feat(blocks): wire Stretch actions and independent reference evaluation

Author a native Stretch action from a Linear, Polar or XY parameter, a two-corner stretch frame, an object selection and a marker. Preview and retain its frame in BEDIT, with selectable and movable action markers.

Freeze line and planar lightweight-polyline vertex bindings, preserve curved-segment height, move wholly selected objects, and evaluate independent saved INSERTs from their definition. Retain native DWG/DXF data, copied-reference isolation and undo/redo.

Validate with synthetic frame, topology, parameter, history, Test Block, drawing-reference, COPY and roundtrip regressions: 340 block tests pass (6 existing opt-in ignored), plus 41 dock and 27 viewport tests. Other entity types, Polar Stretch, custom multipliers and chaining remain unsupported.

src/modules/draw/modify/stretch.rs

@@ -277,7 +277,13 @@ pub fn stretch_entity(
 
 /// The bulge of the segment from vertex `k` once its ends moved from
 /// `before` to `after`: the arc keeps its height, bulge · chord / 2.
-fn held_bulge(bulge: f64, before: &[[f64; 2]], after: &[[f64; 2]], k: usize, closed: bool) -> f64 {
+pub(crate) fn held_bulge(
+    bulge: f64,
+    before: &[[f64; 2]],
+    after: &[[f64; 2]],
+    k: usize,
+    closed: bool,
+) -> f64 {
     let next = if k + 1 < before.len() {
         k + 1
     } else if closed {