Add actor registry

[ZacharyZhang-NY]

Refs #324

This adds a small dramatiq.Registry API for declaring actors before a final broker is available, then binding them once broker configuration is ready.

What changed:

• add Registry.actor(...) as a broker-delayed counterpart to dramatiq.actor(...)
• add Registry.bind(broker=None) to validate actor options against the target broker and declare collected actors
• keep direct synchronous calls available before binding while failing message sending with a clear error until binding
• export Registry from dramatiq
• document the import-before-broker-config use case in the user guide

Validation:

• python -c "import sys, types, pytest; m=types.ModuleType('pylibmc'); m.SomeErrors=Exception; sys.modules['pylibmc']=m; raise SystemExit(pytest.main(['tests/test_registry.py','tests/test_actors.py','tests/test_broker.py','-q','-x']))" -> 43 passed, 3 skipped
• python -m mypy dramatiq/registry.py tests/test_types.py -> success
• python -m black --check dramatiq/registry.py dramatiq/__init__.py tests/test_registry.py tests/test_types.py -> success
• python -m flake8 dramatiq/registry.py tests/test_registry.py -> success
• git diff --check -> success

Local note: Windows cannot build pylibmc here because libmemcached/memcached.h is missing, so the focused pytest run injects a temporary pylibmc module to satisfy tests/conftest.py import. No test stub is committed.