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>
