gpu: enumerate all DRM cards, use lspci for human names

Closes the README 'GPU will not print full name, just generic AMD GPU'
issue. Iterates /sys/class/drm/cardN and runs lspci -d VVVV:DDDD to get
a product name (GeForce RTX 4070 Max-Q / Mobile, Intel Arc Graphics,
...), falling back to the old driver-based name if lspci isn't present.
Also tags cards as [Integrated] or [Discrete] where it's obvious from
the driver (i915/xe vs nvidia/nouveau).