Replace synchronous log writing with zerolog's diode-based non-blocking
writer to prevent logging from blocking the main application during
log bursts. The diode writer buffers up to 1024 messages and logs a
warning when messages are dropped.
- Extract multi-writer logic into separate `multiWriter` function
- Wrap with `diode.NewWriter` for async buffering
- Update both `NewLogger` and `NewLoggerWithFixedLevel` to use diode
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