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/scene/mod.rs

@@ -44,6 +44,7 @@ mod group_layer;
 mod layout;
 mod limits;
 mod modify;
+pub(crate) use modify::remap_copied_reactors;
 mod mspace;
 pub mod viewport_ref;
 pub mod viewport_dimension_pick;