fix(copy): a copy reacts only to what was copied with it

COPY and PASTE kept the source's reactors on each copy, so a copied
dimension still pointed at the source's dimension association (and
boundary or group members at the source hatch or group). Another
application reports such a drawing as damaged and, after recovery,
moves the dimensions. A copy's reactors now follow only targets copied
with it; group copies give their members the new group's reactor; and a
drawing opened with such stray dimension reactors has them dropped.

Fixes #1645
Fixes #1649

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/update/file.rs

@@ -1883,6 +1883,11 @@ impl OpenCADStudio {
         // A file saved without the built-in Standard styles (foreign
         // or damaged) gets them re-seeded so nothing dangles (#366).
         crate::app::style_ops::ensure_standard_styles(&mut self.tabs[i].scene.document);
+        // Copies made by earlier releases kept their source's dimension
+        // association; such a file is reported as damaged elsewhere.
+        crate::scene::dimension_assoc::drop_stray_dimension_reactors(
+            &mut self.tabs[i].scene.document,
+        );
         // Follow the file's saved current UCS from the moment it opens.
         self.tabs[i].adopt_active_ucs_from_header();
         // Adopt the drawing's own Ortho ($ORTHOMODE) and running OSNAP