Add a=sendonly to AES67 SDP for hardware receiver compatibility

[Copilot]

Hardware AES67 receivers (e.g. Nota 142) strictly enforce SDP compliance per AES67-2018 and reject sender sessions missing the a=sendonly direction attribute. Software receivers (VLC, etc.) tolerate its absence, which is why only hardware receivers were affected.

Changes

• pkg/aes67/sdp.go — Added a=sendonly to the media section of BuildSDP(), immediately after the m=audio line:

  m=audio 5004 RTP/AVP 97
  a=sendonly
  a=rtpmap:97 L24/48000/2
  a=ptime:40
  a=ts-refclk:localmac=...
  a=mediaclk:direct=0
  

• pkg/aes67/sdp_test.go — Added a=sendonly to TestBuildSDP_Defaults expected output; added TestBuildSDP_Sendonly to assert the attribute appears in the media section (not the session section).

• README.md — Added hardware receiver troubleshooting section covering the Nota 142 and PTP clock configuration.

Original prompt

software streming empfänger funktioniert hardware empfänger nota 142 noch nicht

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