Raspberry Pi 5 + HUB75 LED matrix panels showing no output — need driver/config debugging

I need help debugging a Raspberry Pi 5 + HUB75 LED matrix display that produces zero output despite the software reporting successful initialization. Setup: • Raspberry Pi 5 with an Adafruit RGB Matrix Bonnet (#3211) • 2×2 grid of Adafruit 64×64 RGB HUB75 panels (P3-64-64-32S-V1.0, 1/32 scan, product #4732), 128×128 total • Currently testing with a single panel isolated • Power confirmed stable at 5.25V via multimeter on all panels — power is ruled out The problem: Running the hzeller/rpi-rgb-led-matrix C++ demo initializes with no errors but shows nothing on any panel — no light, no test pattern, nothing. Already ruled out: • GPIO mapping mode (tried both adafruit-hat and adafruit-hat-pwm) • Chain length settings • The RP1 PIO backend flag • Ribbon/JST cable reseating and a full cable swap • Bonnet’s E-line solder jumper (reflowed with fresh solder) • Bonnet’s GPIO4→18 quality-mode jumper (fully desoldered to rule out a short) Where I am now: Mid-switch to Adafruit’s Pi 5-specific Piomatter Python library (Adafruit-Blinka-Raspberry-Pi5-Piomatter) as an alternative to the hzeller library, per Adafruit’s own Pi 5 docs. Venv and pip install done; about to test a single-panel script. What I need: Someone experienced with HUB75 panels / rpi-rgb-led-matrix / Piomatter on Pi 5 to help diagnose whether this is a driver IC issue, a --led-panel-type config issue, or a defective bonnet — and get at least one panel displaying. Remote debugging via screen share is fine.