Add runtime-checkable Protocol typing to all public interfaces.

Enables structural subtyping for consumers while preserving nominal adapter inheritance, instantiation guards, and scan_keys defaults.

Co-authored-by: Cursor <[email protected]>

README.md

@@ -12,6 +12,8 @@ Subclass an adapter in your own repository to add domain-specific methods while
 | **Adapters**     | Technology-specific base classes (`RedisAdapter`, `MinioAdapter`) |
 | **Your project** | Subclass an adapter and add domain methods                        |
 
+Each public interface is a `@runtime_checkable` `Protocol` with `@abstractmethod` members. **Subclass an adapter** when you need connection management and shared behavior — explicit subclasses get runtime instantiation guards and inherited default methods (e.g. `scan_keys`). **Type-annotate against an interface** when you want loose coupling — any object with the right methods satisfies the contract for mypy and `isinstance()` checks, without inheriting from this package.
+
 Connection adapters expose `connect()`, `disconnect()`, and `is_connected()`. The latter verifies backend reachability with a cached health probe (default TTL: 1 second). Subclasses may override `health_check_ttl_seconds`. `connect()` is idempotent: calling it while already connected and healthy is a no-op.
 
 ## Future direction