media: PcmFrame + AudioSource/Sink + Opus codec pair

PcmFrame is the canonical tap format (16-bit mono @ 24 kHz, 480 samples
per 20 ms frame — ARCHITECTURE.md). AudioSource/AudioSink are the seam
step 2 splices the tap client into (spec §3.3); EchoAudioPipe is the
slice-1 wiring of that seam. OpusDecoder/OpusEncoder wrap the opus
crate's libopus FFI with hot-path match-and-continue (no ? on the 20 ms
loop, spec §3.8); decode/encode return Option<PcmFrame>/Option<Vec<u8>>
so a dropped frame is logged + counted, never propagated to crash the
peer.

Cargo.toml

@@ -6,6 +6,7 @@
 resolver = "2"
 members = [
     "crates/rutster-call-model",
+    "crates/rutster-media",
     "crates/rutster-signaling-sip",
     "crates/rutster-tap",
     "crates/rutster-spend",