Registry schema migrations.
Purpose
-------
Provide a package namespace for runtime registry migrations.
Responsibilities
----------------
- Keep migration modules importable by stable package path.
- Avoid import-time database or filesystem mutation.
Diagnostics
-----------
Domain: OC_CHAT.MAIN
Levels:
L2 - package import lifecycle
L3 - migration package discovery
L4 - import dependency tracing
Contracts
---------
- Migration modules own their own apply and rollback routines.
- This package marker has no side effects beyond diagnostics.
Dependencies
------------
- oc_diagnostics (structured debug logging via _dbg)