[feat](trx-server): wire SoapySDR backend and AudioSource into server startup

- Call validate_sdr() at startup and abort on errors (SDR.md §11)
- Build SoapySdrRig with full [[sdr.channels]] config when access_type is
  "sdr"; subscribe to its primary-channel PCM sender before handing the
  pre-built rig to the rig task via RigTaskConfig.prebuilt_rig
- Skip cpal capture when an SDR audio source is available; bridge the
  SdrPipeline PCM broadcast into pcm_tx so all decoder tasks are unchanged
- Add trx-backend-soapysdr optional dep and soapysdr feature to trx-server
- Add prebuilt_rig field to RigTaskConfig so rig_task skips the registry
  factory when a pre-built rig is supplied by main

Marks SDR-08 complete.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

Cargo.lock

@@ -2525,6 +2525,7 @@ dependencies = [
  "trx-app",
  "trx-aprs",
  "trx-backend",
+ "trx-backend-soapysdr",
  "trx-core",
  "trx-cw",
  "trx-decode-log",