Group lookups (createNodeIfNeeded, findCardNode) must not resolve a
mirrored custom zone that shares the group name. Introduce
findGroupChild to search only non-custom children, and make addCard
consult the deck tree before falling back to creating a top-level zone
so cards added to an un-mirrored custom zone land inside it.
mirrorCustomZones now flattens cards nested at any depth into the
mirrored zone so no card is left without a model row.
Add model behaviour tests (addCard routing, same-name group/zone
collision, removeRows guard, empty-zone survival, findCard inside a
custom zone) and fix the missing main() in the unit test binaries.