feat(sip): add PJSUA2 engine — audio finally reaches the classifier

The gateway can now hear. Verified end to end against the Asterisk lab:
speech classifies as live_human, hold music as music, the speech→music
transition tracks the dialplan, and DTMF reaches a real IVR (Asterisk logged
"caller pressed 1 -> accounts" and branched). RTP stats show 0% packet loss.

PJSUAEngine implements the existing SIPEngine interface, so the gateway,
call manager and hold-slayer service are unchanged. It is selected with
SIP_ENGINE=pjsua2; the default stays "sippy" while this is proven, and the
mock remains opt-in as before.

Why a new engine rather than fixing the old path: PJSUA2 exposes no
standalone RTP media object, so it will not surface media for a dialog it
does not own. Owning the dialog is the price of owning the media. Sippy keeps
the SBC roles it is good at — device registration, routing, leg bridging —
and SippyEngine remains fully functional for signalling; it simply cannot
carry media, which its media branch now says plainly instead of calling a
method that could never work.

The safety invariants are untouched. This engine is reachable only through
gateway.make_call, which refuses emergency numbers and enforces the
concurrency cap before any SIP action. No new dial path was introduced.

MediaPipeline.add_remote_stream(host, port) is replaced by
attach_call_media(stream_id, audio_media), called from onCallMediaState —
the one place PJSUA2 hands out RTP-backed media. Taps requested before media
comes up are attached when it does, so the classifier never misses the start
of a call.

Three crash/lifetime bugs found by running against the real bindings, none of
which any unit test would have caught:

- pj.Call and pj.Account objects finalised after libDestroy() abort the
  process on a native assertion, exactly as media ports do. Both are now
  dropped and collected before the pipeline destroys the endpoint.
- PJSUA2 keeps delivering callbacks during interpreter teardown, when module
  globals may already be cleared. The callbacks alias what they need locally
  and swallow everything: a raise there escapes into C++ and takes the worker
  thread with it.
- hangup() only queues the BYE, so shutdown deleted the account with a call
  still active and left the far end on an unclosed dialog. stop() now waits
  briefly for the teardown to complete.

Threading follows the established rule: PJSUA2 worker threads reach the loop
only through _post_from_pj → run_coroutine_threadsafe, and any thread PJSUA2
did not create registers itself before touching a PJSUA2 object.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/sippy_engine.py

@@ -276,17 +276,22 @@ class SippyEngine(SIPEngine):
             if state == "connected":
                 sdp = data.get("sdp")
                 if sdp and self.media_pipeline:
+                    # Signalling only: PJSUA2 surfaces RTP media exclusively
+                    # through a call it owns, so a Sippy-owned dialog can
+                    # never be given media — the classifier stays deaf on this
+                    # engine. PJSUAEngine is the media-capable path; see
+                    # docs/architecture.md. Log the negotiated endpoint so the
+                    # SIP exchange is still debuggable.
                     try:
                         remote_rtp = self._parse_sdp_rtp_endpoint(sdp)
                         if remote_rtp:
-                            leg.media_port = self.media_pipeline.add_remote_stream(
-                                leg.leg_id,
-                                remote_rtp["host"],
-                                remote_rtp["port"],
-                                remote_rtp["codec"],
+                            logger.info(
+                                f"  {leg.leg_id}: remote RTP "
+                                f"{remote_rtp['host']}:{remote_rtp['port']} "
+                                f"({remote_rtp['codec']}) — no media on this engine"
                             )
                     except Exception as e:
-                        logger.error(f"  Failed to set up media for {leg.leg_id}: {e}")
+                        logger.error(f"  Failed to parse SDP for {leg.leg_id}: {e}")
             elif state == "terminated":
                 if self.media_pipeline and leg.media_port is not None:
                     try: