Replace hardcoded "rtl_fm" references in wefax.py with the actual SDR
tool name so error messages correctly show "rx_fm" for non-RTL devices.
Use get_tool_path('rx_fm') in all SoapySDR command builders to match
the pattern already used for rx_sdr.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>