Fix Nota142 AES67 audio: add a=sendonly to SDP, fix SAP messageID churn

[Copilot]

The Nota142 (and compatible AES67 hardware receivers) were not discovering the stream via SAP and produced no audio even when the SDP was added manually.

Changes

• pkg/aes67/sdp.go — Add a=sendonly
  AES67 hardware receivers use the media direction attribute to gate audio decoding. Without it the stream appears in the UI but is silent. Placed between a=ptime and a=ts-refclk per common AES67 SDP convention:

  a=ptime:40
  a=sendonly
  a=ts-refclk:localmac=…
  

• pkg/aes67/sap.go — Remove messageID++ from send()
  RFC 2974 requires all periodic re-announcements of the same session to carry the same message ID. Incrementing it on every 30 s tick made each announcement look like a new session to SAP listeners, preventing stable stream discovery. The ID is now fixed for the session lifetime (randomly seeded at construction, as before).

• pkg/aes67/sdp_test.go — TestBuildSDP_Defaults extended to assert a=sendonly is present.

Original prompt

leider noch keine audio im Nota142 adapter stream wird auch nicht angezeigt manuelees hinzufügen mit v=0
o=- 0 0 IN IP4 192.168.72.204
s=gostreamer
c=IN IP4 239.69.250.171/32
t=0 0
m=audio 5004 RTP/AVP 97
a=rtpmap:97 L24/48000/2
a=ptime:40
a=ts-refclk:localmac=D8-3A-DD-19-A7-F5
a=mediaclk:direct=0
geht aber auch ohne Ton

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