Check the machine before installing, and keep old shells' mise working

install.sh now stops early, with a clear message, if this isn't 64-bit
Arch, it's run as root, there's no internet, or sudo doesn't work.
system/preflight.sh then asks before going on with too little disk space
or / not on btrfs, and offers to switch another login screen to GDM and
PulseAudio to PipeWire. Keeping another login screen puts GDM in
services-off, so install and update stop failing to enable it; a
migration does the same on existing machines. It says nothing when all
is well, as refresh runs install.sh again.

Intel GPUs also get intel-media-driver for hardware video decoding, with
a migration for existing machines.

The mise migration links ~/.local/bin/mise to /usr/bin/mise instead of
deleting it, so shells opened before the update don't print an error at
every prompt.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/preflight.bats

@@ -0,0 +1,122 @@
+#!/usr/bin/env bats
+
+load test_helper
+
+# A machine where all is well: lots of space, / on btrfs, GDM or no login
+# screen yet, no PulseAudio. Tests change one thing each.
+setup() {
+  common_setup
+  mkdir -p "$HOME/.local/share"
+  ln -s "$REPO" "$HOME/.local/share/coredesktop"
+  export FREE_KIB=$((200 * 1024 * 1024)) FSTYPE=btrfs DM="" INSTALLED=""
+  stub df 'printf "Avail\n%s\n" "$FREE_KIB"'
+  stub findmnt 'echo "$FSTYPE"'
+  stub readlink '[ -n "$DM" ] && echo "/usr/lib/systemd/system/$DM"'
+  stub pacman '
+case "$1" in
+  -Qi) [[ " $INSTALLED " == *" $2 "* ]] ;;
+  -Qq) for p in $INSTALLED; do echo "$p"; done ;;
+esac'
+  stub sudo
+  # No terminal unless a test gives answers
+  export COREDESKTOP_TTY="$BATS_TEST_TMPDIR/no-tty/tty"
+}
+
+answer() {
+  export COREDESKTOP_TTY="$BATS_TEST_TMPDIR/answer"
+  echo "$1" > "$COREDESKTOP_TTY"
+}
+
+already_installed() {
+  mkdir -p "$HOME/.local/state/coredesktop/migrations"
+}
+
+@test "preflight says nothing and changes nothing when all is well" {
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+  not_called sudo
+}
+
+@test "preflight stops on too little disk space unless told to go on" {
+  export FREE_KIB=$((30 * 1024 * 1024))
+  answer n
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 1 ]
+  [[ "$output" == *"Only 30 GB is free"* ]]
+
+  answer y
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+}
+
+@test "preflight needs less space on a machine that's already set up" {
+  export FREE_KIB=$((30 * 1024 * 1024))
+  already_installed
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "preflight warns once that / isn't btrfs, and goes on without a terminal" {
+  export FSTYPE=ext4
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"/ is on ext4, not btrfs"* ]]
+
+  already_installed
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  [ -z "$output" ]
+}
+
+@test "preflight switches another login screen to GDM when asked to" {
+  export DM=sddm.service
+  answer y
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  called sudo systemctl disable sddm.service
+}
+
+@test "preflight keeps another login screen, and update then leaves GDM off" {
+  export DM=sddm.service
+  answer n
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  not_called sudo
+  grep -qx "system gdm.service" "$HOME/.config/coredesktop/services-off"
+
+  # Not asked again
+  run bash "$REPO/system/preflight.sh"
+  [ -z "$output" ]
+}
+
+@test "preflight changes no login screen without a terminal to ask on" {
+  export DM=sddm.service
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  not_called sudo
+}
+
+@test "preflight replaces PulseAudio and its modules with PipeWire when asked to" {
+  export INSTALLED="pulseaudio pulseaudio-bluetooth"
+  answer y
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  called sudo pacman -R --noconfirm pulseaudio-bluetooth
+  called sudo pacman -S --needed --noconfirm --ask=4 pipewire pipewire-pulse pipewire-alsa wireplumber
+}
+
+@test "preflight keeps PulseAudio when told to, or without a terminal" {
+  export INSTALLED="pulseaudio"
+  answer n
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  [[ "$output" == *"Keeping PulseAudio."* ]]
+  not_called sudo
+
+  export COREDESKTOP_TTY="$BATS_TEST_TMPDIR/no-tty/tty"
+  run bash "$REPO/system/preflight.sh"
+  [ "$status" -eq 0 ]
+  not_called sudo
+}