Add IFlushableLogSink interface and LogManager.FlushAsync

[DJGosnell]

Summary

• Closes IFlushableLogSink interface and LogManager.FlushAsync #9
• Adds IFlushableLogSink opt-in interface for sinks that support mid-stream flushing
• Implements flush via TCS sentinel pattern in BufferedLogSink
• Adds LogManager.FlushAsync(TimeSpan?) to flush all flushable sinks

Reason for Change

There was no way to force-flush buffered sinks without shutting down. After a critical error, an application may need to ensure all buffered log entries are written to disk/network before attempting recovery — without tearing down the logging pipeline.

Impact

• BufferedLogSink (and its subclasses FileSink, StreamSink) now implement IFlushableLogSink
• Non-buffered sinks (ConsoleSink, DebugSink, RecordingSink) are unaffected — they don't implement the interface and are simply skipped during flush
• LogManager.FlushAsync() iterates all sinks, calling FlushAsync only on IFlushableLogSink implementations

Migration Steps

None required — this is purely additive. Existing code continues to work unchanged.

Performance Considerations

• Flush uses a TCS sentinel enqueued into the existing channel — zero additional locks or synchronization primitives
• CreateFlushSentinel uses Array.Empty<byte>() to avoid allocation for the unused buffer field
• TaskCreationOptions.RunContinuationsAsynchronously prevents inline continuation on the drain loop thread
• No performance impact on normal (non-flush) write paths — the IsFlushSentinel check is a single null reference check

Security Considerations

None.

Breaking Changes

• Consumer-facing: None
• Internal: BufferedEntry record struct gains an optional FlushCompletion parameter (default null) — source-compatible for existing derived sinks