3 failing tests in tests/test_vendor_capabilities.py that establish the
core behaviors of the new VendorCapability matrix:
1. test_registry_lookup_known_model: registering and looking up a specific
(vendor, model) entry returns the registered entry
2. test_fallback_to_vendor_default: looking up an unregistered model returns
the vendor's '*' default entry
3. test_unknown_vendor_raises: looking up a vendor with no entries raises
KeyError with a 'No capabilities registered' message
All 3 tests fail with ModuleNotFoundError: No module named
'src.vendor_capabilities' (confirmed via pytest). The implementation file
will be created in the next commit (Green phase).
The autouse _clean_registry fixture snapshots src.vendor_capabilities._REGISTRY
before each test and restores it after, providing test isolation for the
module-level state.
