New files:
- mhwd-nvidia-hybrid-live: auto-detect and install NVIDIA hybrid
  driver on live boot via mhwd, with modprobe fallback, DRI
  permissions fix, switcheroo activation, and drirc creation
- mhwd-nvidia-hybrid-live.service: systemd unit that runs before
  display-manager, gated by ConditionPathExists=/livefs-pkgs.txt

Fixes in biglinux-gpu-manager:
- Replace all bbv_change_variable calls with direct sed commands
  (bbv_change_variable binary does not exist)
- Rename addInSddm to addInDisplayManager with support for SDDM,
  LightDM, and graceful skip when no display is available
- Add file existence guards before sed/grep on optional config
  files (grub, nvidia.conf, xorg conf, sddm Xsetup)
- Add mkdir -p /etc/modprobe.d before writing nvidia.conf
- Add mhwd availability check in autoInstallDriver

Fixes in mhwd-verify-hybrid-boot:
- Replace 16-branch elif/grep chain with single grep -oP parser
- Add error handling and guards on all system calls
- Remove unnecessary sudo call

Updated mhwd-biglinux.install:
- Enable/disable mhwd-nvidia-hybrid-live.service on install/remove