Touchscreen runner

Configures the project’s touchscreen package and session-hook layer. Modes are preflight, apply, and disable.

Touchscreen hook with identity mapping

wget -qO- https://devs-guide.github.io/debian/setup/cli/touchscreen.sh | \
  env DEBIAN_TOUCHSCREEN_MODE=apply \
      DEBIAN_TOUCHSCREEN_ENABLE=1 \
      DEBIAN_TOUCHSCREEN_USER=app \
      DEBIAN_TOUCHSCREEN_INSTALL_PACKAGES=1 \
      DEBIAN_TOUCHSCREEN_MATCH='eGalax|D-WAV|Titan6001|TouchController|Touchscreen' \
      DEBIAN_TOUCHSCREEN_MATRIX=identity \
      DEBIAN_TOUCHSCREEN_INSTALL_EVTEST=1 \
      bash

Variable	Purpose
`DEBIAN_TOUCHSCREEN_MATCH`	Regular expression used to select the intended input device. Review against `xinput` output on the target.
`DEBIAN_TOUCHSCREEN_PRIMARY_ID` / `DEBIAN_TOUCHSCREEN_DISABLE_IDS`	Optional explicit X input IDs; do not persist IDs until the host inventory is known.
`DEBIAN_TOUCHSCREEN_OUTPUT`	Optional display output to map the selected device to.
`DEBIAN_TOUCHSCREEN_MATRIX`	Uses `identity` here; use `custom` only with a reviewed `DEBIAN_TOUCHSCREEN_CUSTOM_MATRIX`.
`DEBIAN_TOUCHSCREEN_INSTALL_EVTEST`	Installs `evtest` for device diagnostics.

Device matching, output selection, and matrices are controlled through DEBIAN_TOUCHSCREEN_* variables. Keep device-specific calibration changes explicit and retain a tested configuration for the target display.