add_observer used to be a no-op when no RTP transport existed yet (a
WebRTC transport only appears after ICE selects a pair; direct RTP
creates one while applying SDP), forcing consumers to poll-and-reattach.
Any inbound packet in that window — most painfully the first RFC 4733
telephone-event (DTMF) right after answer — was invisible to stats /
DTMF detection / recording.

Observers are now remembered on the PeerConnection and re-attached at
every transport creation site (start_dtls, SDES re-setup, muxed media
transports). Regression-tested with an observer registered before the
transport exists (fails when the re-attach hook is disabled).