The test-double AudioPipe that simulates a caller. Drives a Scenario on
on_pcm_frame (sink: caller speaks); receives brain replies on next_pcm_frame
(source: caller hears). Both timestamps anchored to Instant::now() inside
this pipe -- the harness cannot lie about latency because the only clock it
uses is the caller's (spec section 2.2).
Capture enum carries CallerLoudOnset / BargeKillObserved / CallerHeardReply
timestamps. BargeKillObserved is captured unconditionally on empty
reply_ring -- the LatencyProbe (S3) dedups captures without a prior onset,
keeping the hot path branch-free.
LatencyProbe (S3) consumes the Capture stream post-run.
Signed-off-by: Aaron D. Lee <himself@adlee.work>