Fix Back passthrough on webOS 25

[afonsojramos]

Summary

• Use LGE M-RCU - Builtin [1] for exclusive passthrough on webOS 25.
• Normalize the Linux Back event (code 412) into one fresh, complete keypress.
• Preserve the existing passthrough behavior on webOS 9 and earlier.

Problem

On webOS 25, relaying remote events through the existing Builtin [2] output changes Back behavior: pressing Back inside a nested menu exits the entire app instead of closing the menu.

This affected every app while Magic Mapper was running in exclusive mode. Stopping Magic Mapper restored native Back behavior.

Hardware validation

Tested on an LG C3 running webOS 25 / Rockhopper 10.3.1-3006:

• Native Back closed only the nested menu.
• Existing exclusive passthrough through [2] exited the app.
• A clean Back press through [1] closed only the menu while [0] was exclusively grabbed.
• With the complete patch running persistently:
  • Back closed only the nested menu.
  • Arrow and OK input responded immediately afterward.
  • Configured Netflix and Rakuten buttons remained blocked.

python3 -m py_compile magic_mapper.py also passes.