Add Stratum reconnect logic to recover from BrokenPipeError

[Welshdag]

Summary

• Reconnects to the Stratum pool with exponential backoff when the connection drops (BrokenPipeError, EOF, or other socket errors)
• Re-subscribes/re-authorizes after reconnecting, and retries an in-flight mining.submit once after reconnecting
• Reader thread now reconnects on read errors or pool-initiated disconnects instead of leaving the miner in a stuck "connected" state

Test plan

• python3 -m py_compile bdag_gpu_miner/gpu_scrypt_miner.py
• Manual test against a local fake Stratum server: dropped the connection mid-submit, confirmed the miner reconnects, re-authorizes, and the retried submit is accepted
• tests/test_hashing.py still passes