docs(media): add reflex module-map entry to lib.rs catalogue (slice-4 §3.1)

Task 1 review flagged the new reflex module was missing from the
module-map //! catalogue. AGENTS.md learner-facing convention requires
the catalogue entry describing what the module does + why it exists.

Addresses the Minor finding from the Task 1 task review.

Signed-off-by: opencode controller <controller@local>

crates/rutster-media/src/lib.rs

@@ -27,6 +27,8 @@
 //! - [`pcm`] — `PcmFrame` + `AudioSource`/`AudioSink` traits (the tap
 //!   seam) + `EchoAudioPipe` (slice-1 wiring).
 //! - [`opus_codec`] — `OpusDecoder`/`OpusEncoder` wrappers.
+//! - [`reflex`] (slice-4) — `AdvisoryEvent` + `ReflexMetrics`; the FOB
+//!   barge-in reflex state machine will decorate `AudioPipe` here (Task 2).
 //! - [`loop_driver`] (Task 4) — the str0m poll loop on tokio.
 //! - [`rtc_session`] (Task 4) — `RtcSession`, the per-peer owner.