feat: modem restart detection via error counter reset (#60)

[itsDNNS]

Summary

Detects modem restarts by observing per-channel DOCSIS error counter resets between consecutive polling cycles. Emits modem_restart_detected event to Event Log.

• _check_restart() in EventDetector, before _check_errors()
• Per-channel DS counter comparison (>=80% declining threshold)
• Continuity guard against channel map changes (>=50% overlap, min 4 channels)
• Summary total sanity check
• Relaxed declining definition: at least one counter type declined, neither increased

Test plan

• 13 unit tests covering detection, thresholds, edge cases
• Full test suite passes
• i18n validation passes
• No false positives on normal operation (counters increasing)
• No false positives on channel map changes
• Correctly handles (N, 0) -> (0, 0) scenario
• Does not trigger error_spike on restart