coder / slog / a4f439e455e1db7c96c91b1c24bd5ca561e2df89

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feat: add FlightRecorder sink for deferred low-level logging (#237)

* feat: add Buffer sink for deferred low-level logging

Buffer is a Sink that holds entries below a configured level in a
fixed-size in-memory ring, forwarding entries at or above the level
immediately. Flush replays the buffered entries to the wrapped sinks.

This lets a program run its logger at LevelDebug to capture verbose
entries while only emitting them on demand (e.g. on an error or
connection failure), with a configurable buffer size. When full, the
oldest entry is evicted.

* feat: add Flusher interface for deferred sinks

Flusher (Flush(ctx)) lets callers trigger a flush without depending on
the concrete Buffer type, which is handy when the buffer is created in
one place and flushed in another. Buffer implements it.

* refactor: rename buffer.go and buffer_test.go to flightrecorder.go and
flightrecorder_test.go

* refactor: rename Buffer to FlightRecorder

* refactor: make flight recording a Logger flag with auto-flush on error

Rework the flight recorder from a standalone Buffer/Flusher sink into a
Logger capability. Logger.FlightRecorder(size) enables a rolling
in-memory history of below-level entries; the history is flushed to the
sinks automatically when an entry at LevelError or above is logged, or
manually via Logger.Flush.

Enabling flight recording is independent of the Logger level, so it is
order-independent with Leveled(). Derived loggers (With, Named, Leveled)
share the underlying ring and capture their derived fields and names.

Replace the fill-tracking branch in the ring with a monotonic written
counter: the ring is allocated at full length and writes always land at
written%size, so the write path is branchless and drain derives the
entry count and oldest position from written.

* refactor: track ring position and length instead of unbounded counter

Replace the monotonic written counter with bounded pos and length fields,
so neither grows without limit and t... (continued)

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