HMC472ASerial class implements usb-serial-json-v1 protocol over the
ESP32-S3's native USB CDC port. Auto-detection scans /dev/ttyACM* and
probes with the identify command to find the right port.
--attenuator flag now defaults to 'auto' (USB first, HTTP fallback).
Also accepts direct serial port paths or HTTP URLs for explicit control.
New tool (tools/rf_testbench.py) automates five test sequences using a
NanoVNA as a CW source and HMC472A digital attenuator (0-31.5 dB, 0.5 dB
steps via REST API) to characterize the SkyWalker-1 receiver:
- AGC linearity mapping across 64 attenuation steps
- IF band flatness sweep (950-1500 MHz)
- Frequency accuracy via peak detection
- Minimum detectable signal search
- BPSK mode 9 CW probe (Viterbi rate 1/2 K=7)
Includes SKYWALKER_MOCK=1 mode, path-loss calibration from NanoVNA S21
sweeps, and safe-state cleanup (attenuator to max on exit, LNB power
never enabled in direct-input mode).
Also adds Applications & Use Cases guide, RF Test Bench docs page, fixes
h21cm cable loss (was 3x too high), and updates sidebar.
