migrateV1ToV2 assigned each entry id via generateId(ids) but never added the
result back into ids, so the collision-check set stayed empty for the whole
migration and generateId's check was a no-op. A v1 to v2 upgrade could then mint
two entries with the same 8-hex id, and because the migration rebuilds the
parent/child tree from those ids it would parent the second entry to itself,
corrupting the branch. Add ids.add(entry.id) so the generator sees prior ids and
retries on collision.
Adds a regression test that drives the real SessionManager.open migration path
with a seeded id collision.
4b4211e6c7