G3: N11 — document transport connect() return contract

The three transports had inconsistent connect() return contracts:
TCP returned an asyncio.Future (fixed by F01), serial returned
self.port (always truthy), BLE returned self.address or None.
ConnectionManager's success check `if result is not None:` was
tautological for TCP and serial.  With F01 fixed, the check is now
meaningful for all three.

Add a comprehensive docstring to ConnectionProtocol documenting the
contract: return truthy on success (included in CONNECTED event
payload), return None for soft failure (retry), or raise for hard
failure (also retry, logged).  Also import Awaitable for the F02
reconnect_callback type hint that follows.

Refs: Forensics report finding N11

src/meshcore/connection_manager.py

@@ -4,14 +4,23 @@ Connection manager that orchestrates reconnection logic for any connection type.
 
 import asyncio
 import logging
-from typing import Optional, Any, Callable, Protocol
+from typing import Optional, Any, Awaitable, Callable, Protocol
 from .events import Event, EventType
 
 logger = logging.getLogger("meshcore")
 
 
 class ConnectionProtocol(Protocol):
-    """Protocol defining the interface that connection classes must implement."""
+    """Protocol defining the interface that connection classes must implement.
+
+    Return contract for connect():
+        - On success: return a truthy value (typically an address string)
+          that identifies the connection. This value is included in the
+          CONNECTED event payload as ``connection_info``.
+        - On failure: return ``None`` (soft failure — triggers a retry in
+          ``_attempt_reconnect``) **or** raise an exception (hard failure —
+          also triggers a retry, logged as an error).
+    """
 
     async def connect(self) -> Optional[Any]:
         """Connect and return connection info, or None if failed."""