The branch compare panel now offers per-change selection, grouped by
the component each shape belongs to. Every summary entry carries a
stable change key (kind/page/id, the design-branches PoC format), and
the panel's checkboxes key on it.
merge-file-branch and update-branch-from-main take the deselected keys
as an excluded set. An excluded change is dropped on the user's word:
a conflicting entity resolves to the target side (the recorded choice,
per the no-silent-winner decision), and an excluded addition cascades
to its added descendants, so no emitted change references a parent the
merge does not create. A merge whose every change is excluded refuses
with :nothing-to-merge.
The update squash needs no filter: whatever excluded main work is
missing from the branch lands in the squashed net as a branch-side
delta, so the branch state still replays from its repositioned base.
AI-assisted-by: glm-5.3-flash
Signed-off-by: Álvaro Tejero Cantero <alvorithm@teje.ro>
fe60e1a772