Fix Amlogic S19j Pro runtime bring-up

[benjamin-wilson]

Title

Fix Amlogic S19j Pro runtime bring-up

Base

• Repo: skot/mujina
• Base branch: amlogic-s19jpro
• Head branch: codex/amlogic-s19jpro-runtime-fixes

Summary

• fix the native S19j Pro Amlogic hashboard sensor and EEPROM bus mapping
• tolerate missing APW12 control traffic by continuing in GPIO-only PSU-enable mode
• make the BM1362 target frequency configurable through MUJINA_TARGET_FREQ_MHZ
• load runtime settings from /home/root/mujina.env and add an example env file
• update README bring-up notes to match the corrected HB2 mapping and env-based startup flow

Testing

• git diff --check
• bash -n start.sh
• rebuilt mujina-minerd successfully via the loader cross-build flow
• flashed and verified on live S19j Pro Amlogic hardware
  • corrected HB2 temp/EEPROM reads
  • board registration and mining on HB2
  • verified MUJINA_TARGET_FREQ_MHZ=200 and =500 both take effect at runtime