feat: fix brightness in every GPU mode and add zephyrus-backlight.py

Integrated mode needs acpi_backlight=native amdgpu.backlight=0, but that alone breaks Hybrid and Ultimate, where brightness goes through the EC. A modprobe rule now forces nvidia_wmi_ec_backlight only while the NVIDIA dGPU is on the PCI bus, verified after real reboots in Integrated and Hybrid.

The known-issues entries are rewritten around the verified root cause, with terminal evidence, diagrams and a screenshot. zephyrus-backlight.py applies, removes, tests and reports on the fix from a zenity menu or the terminal. Ultimate mode is still untested.

Author: Stensel8