[fix](trx-backend-soapysdr): improve device initialization diagnostics

Enhance error handling and messaging for SoapySDR device initialization:
- Add fallback logic to try empty args if device-specific args fail
- Provide detailed multi-line error messages with troubleshooting guidance
- Include suggestions to check SoapySDR plugins and run SoapySDRUtil --probe
- Clarify device lifetime management in struct
- Document why actual streaming not yet implemented (soapysdr 0.3 limitations)
- Note that sdr 0.3 requires FFI or upgrade for streaming support

This helps users diagnose device initialization failures and understand
the current architectural limitations.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

test-airspy.toml

@@ -0,0 +1,26 @@
+rigs = [
+  { id = "airspy0", model = "soapysdr", name = "AirspyHF" }
+]
+
+[general]
+callsign = "N0CALL"
+log_level = "debug"
+
+[listen]
+enabled = true
+listen = "127.0.0.1"
+port = 4530
+
+[listen.auth]
+tokens = []
+
+[audio]
+enabled = true
+listen = "127.0.0.1"
+port = 4531
+rx_enabled = true
+sample_rate = 48000
+
+[airspy0.access]
+type = "sdr"
+args = "driver=AirspyHF,serial=c852eb5dd23539f8"