WS `_recv_loop` reaches into private `ConnectionManager._set_state`

[TexasCoding]

From Wave 5 architecture audit, finding F-N-06. Severity: medium.

Code

`kalshi/ws/client.py:196`:
```python
await self._connection._set_state(ConnectionState.STREAMING)
```

After a successful reconnect, `KalshiWebSocket._recv_loop` reaches into `ConnectionManager`'s name-mangled `_set_state`. Everything else on `ConnectionManager` treats state transitions as internal.

Why it's a problem

• Leaky abstraction — the client pokes the manager's state machine.
• A refactor that renames or removes `_set_state` silently breaks the streaming state report.
• The `on_state_change` callback fires from the "wrong" caller.

Fix

Either:

• Add a public `mark_streaming()` (or similar) on `ConnectionManager` that performs the same transition.
• Have `ConnectionManager.reconnect()` perform the CONNECTED → STREAMING transition itself after resubscribe so the client doesn't need to.

The second option moves the transition where the rest of the state machine lives.