refactor(installer): extract validate_yesno and a NVIDIA driver-floor constant

Four near-identical blocks validated that AUTOLOGIN, NO_GPU_DRIVERS, INSTALL_CLAUDE_CODE, and INSTALL_DEVICE_UDEV_RULES were empty or exactly yes/no. They collapse into a validate_yesno helper the callers guard with `|| exit 1`.

The NVIDIA driver floor of 535 was a magic literal in the comparison and three prose lines. It now lives in a NVIDIA_MIN_DRIVER constant, and the nvidia-preflight test sources that constant alongside the function it extracts.

This is the safe, purely-testable slice of the refactor rollup. The structural extractions that touch the installer's system-mutation paths -- the GPU modalias scan, the stow and retry loops, the display-server/window-manager dispatch, and the snapper/fsck/GRUB/waybar sed blocks -- are split into a follow-up so they get characterization coverage first rather than a blind rewrite.

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