[feat](trx-server): add soapysdr as default feature

Enable SoapySDR backend by default to make multi-device setups work out
of the box without requiring explicit feature flags.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/trx-server/Cargo.toml

@@ -9,6 +9,7 @@ edition = "2021"
 build = "build.rs"
 
 [features]
+default = ["soapysdr"]
 soapysdr = ["dep:trx-backend-soapysdr", "trx-backend/soapysdr"]
 
 [dependencies]