Part of #77.
Do NOT gate append (would lose history for muted observers in groups).
- Reject non-member transport.
- Append member messages unconditionally.
- Gate WAKE/response only by owner/addressee.
AC: muted observer message is stored but does not wake; non-member transport rejected.
Part of #77.
Do NOT gate append (would lose history for muted observers in groups).
AC: muted observer message is stored but does not wake; non-member transport rejected.