Code-review MAJOR MA3. The initial Alembic migration imported the live
`mimic.rbac.matrix.GROUP_PERMISSIONS` to seed the `permission` / `group` /
`group_permission` rows. That breaks the Alembic invariant "a migration
produces the same schema regardless of when you replay it": a future tweak
to the runtime matrix would silently change the seeded baseline on a fresh
DB.
Two changes:
1. The migration now carries an *inline frozen snapshot* of the F11 matrix
(`_PERMISSIONS_FROZEN`, `_GROUP_PERMISSIONS_FROZEN`, `_GROUP_DESCRIPTIONS`).
The seed reads from these tuples/dicts only. If the canonical matrix
evolves, the next migration is responsible for the delta.
2. A new unit test `test_migration_seed_matches_current_matrix` enforces
that the frozen seed equals the runtime `Permission` enum and
`GROUP_PERMISSIONS` mapping. Drift now fails CI loudly with a hint to
write a new migration instead of editing the existing one.
Also: docstring no longer mentions `ttp_version` (M8 follow-up).
