[feat](trx-backend-soapysdr): add crate scaffold with RigCat stub

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>

Cargo.toml

@@ -16,6 +16,7 @@ members = [
     "src/trx-server/trx-backend",
     "src/trx-server/trx-backend/trx-backend-ft817",
     "src/trx-server/trx-backend/trx-backend-ft450d",
+    "src/trx-server/trx-backend/trx-backend-soapysdr",
     "src/trx-client",
     "src/trx-client/trx-frontend",
     "src/trx-client/trx-frontend/trx-frontend-http",