Introduces the trx-backend-soapysdr crate with a compilable SoapySdrRig
struct that satisfies the Rig + RigCat trait bounds. RX methods
(get_status, set_freq, set_mode, get_signal_strength) are implemented;
TX-only methods return RigError::not_supported. as_audio_source returns
None for now (overridden in SDR-07). Wires the crate into the workspace
and trx-backend (feature "soapysdr"), and fixes the non-exhaustive match
on RigAccess::Sdr in trx-server main.rs and rig_task.rs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
