[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/trx-backend-soapysdr/Cargo.toml

@@ -0,0 +1,22 @@
+# SPDX-FileCopyrightText: 2025 Stanislaw Grams <stanislawgrams@gmail.com>
+#
+# SPDX-License-Identifier: BSD-2-Clause
+
+[package]
+name = "trx-backend-soapysdr"
+version = "0.1.0"
+edition = "2021"
+license = "BSD-2-Clause"
+
+[dependencies]
+trx-core = { path = "../../../trx-core" }
+tokio = { workspace = true, features = ["sync", "rt"] }
+serde = { workspace = true }
+tracing = { workspace = true }
+num-complex = "0.4"
+# soapysdr is an optional system-library dep gated behind a feature
+soapysdr = { version = "0.3", optional = true }
+
+[features]
+default = []
+soapysdr-sys = ["dep:soapysdr"]