[feat](trx-backend): add RigAccess::Sdr variant and soapysdr feature stub

Add `RigAccess::Sdr { args: String }` to the backend access enum, register
a feature-gated `soapysdr` factory stub in `register_builtin_backends_on`,
handle the new variant in all existing `match` arms, and add the `soapysdr`
feature flag (no dep yet; implementation lands in SDR-04). Mark SDR-02 done.

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

@@ -11,6 +11,7 @@ edition = "2021"
 default = ["ft817", "ft450d"]
 ft817 = ["dep:trx-backend-ft817"]
 ft450d = ["dep:trx-backend-ft450d"]
+soapysdr = []   # implementation wired in SDR-04
 
 [dependencies]
 trx-core = { path = "../../trx-core" }