fix(rpi): stop the kiosk login loop by pinning the vc4 display device

[danisss9]

On a Pi, dtoverlay=vc4-kms-v3d registers two DRM devices: vc4, which owns
the HDMI connectors, and v3d, a render-only core with no outputs. Their
/dev/dri/card* numbering is not stable, and when v3d lands on card0 Xorg's
modesetting driver binds to it, finds no screens and exits immediately.

The console is unaffected - fbcon is bound to vc4 - so the machine boots,
prints the banner, and then loops: startx was exec'd from .bash_profile, so
a failed session ended the login shell, agetty logged the user straight back
in, and the only explanation went to a log file inside a session that never
lasted long enough to read it.

• Add the vc4 OutputClass/PrimaryGPU rule to the Pi overlay, matching on the
  kernel driver name rather than a card number. Raspberry Pi OS ships the
  same rule; Arch Linux ARM does not.
• Run startx as a child of the login shell instead of exec'ing it. A session
  that lived logs out as before; three failures inside a few seconds stop the
  retries and leave a shell on tty1 with the tail of the session and kiosk
  logs on screen.
• Fall back to software rendering when the editor exits immediately three
  times, and detect a usable DRM node instead of just /dev/dri existing.
• Assert the vc4 rule made it into the image at build time, and document the
  console fallback.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01Mej775L1dYS9Hk6JtvcHCJ