Control frames not marked as lost on old path after migration

After migration, the server may have unacknowledged packets on the old path carrying control frames that are never retransmitted.

Neither retransmission mechanism covers these frames:

**ACK-based** loss detection does not fire on the old path.
On the old path, `newly_acked` is always empty so `on_ack_received` returns early without calling `detect_lost_packets`.

**PTO-based** retransmission skips control frames.
The PTO filter only selects packets where `has_data` is true, so control frames are excluded.

Affected frames are: `RESET_STREAM`, `STOP_SENDING`, `MAX_STREAM_DATA`, `MAX_DATA`, `MAX_STREAMS`, `RETIRE_CONNECTION_ID`, `NEW_CONNECTION_ID`

This can lead to **indefinite stalls**.

I will share my unit tests soon.