Adds RingBufferFallback — an in-memory drop-oldest ring buffer used by
the upcoming FallbackAuditWriter (Bundle B-T4) when the primary SQLite
writer is throwing. Backed by Channel<AuditEvent> with
BoundedChannelFullMode.DropOldest, fixed capacity (default 1024).
Channel.CreateBounded(DropOldest) does NOT natively signal a drop on
TryWrite, so overflow is detected by comparing Reader.Count before and
after the enqueue: when the buffer is already at capacity and a new
TryWrite succeeds while keeping the count at capacity, exactly one
event was displaced and RingBufferOverflowed is raised (one event per
drop).
Public surface:
- bool TryEnqueue(AuditEvent) — always succeeds unless completed.
- IAsyncEnumerable<AuditEvent> DrainAsync(CancellationToken) — FIFO.
- void Complete() — closes the channel so DrainAsync can finish.
- event Action? RingBufferOverflowed — health counter hook.
Tests (3 new, total 23 -> 26):
- Enqueue_1025_Into_1024Cap_Ring_DropsOldest_AndRaisesOverflowOnce
- DrainAsync_Yields_FIFO_Then_Completes_When_Empty
- TryEnqueue_AllSucceeds_ReturnsTrue
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