[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>

src/trx-server/trx-backend/Cargo.toml

@@ -11,12 +11,13 @@ edition = "2021"
 default = ["ft817", "ft450d"]
 ft817 = ["dep:trx-backend-ft817"]
 ft450d = ["dep:trx-backend-ft450d"]
-soapysdr = []   # implementation wired in SDR-04
+soapysdr = ["dep:trx-backend-soapysdr"]
 
 [dependencies]
 trx-core = { path = "../../trx-core" }
 trx-backend-ft817 = { path = "trx-backend-ft817", optional = true }
 trx-backend-ft450d = { path = "trx-backend-ft450d", optional = true }
+trx-backend-soapysdr = { path = "./trx-backend-soapysdr", optional = true }
 tokio = { workspace = true, features = ["full"] }
 tokio-serial = { workspace = true }
 serde = { workspace = true, features = ["derive"] }