chore(packaging): add AUR package sources for source, git, and binary builds

packaging/aur/ now holds the full content of three AUR repositories:
proton-drive-for-linux builds the latest tag from source,
proton-drive-for-linux-git builds main, and proton-drive-for-linux-bin
installs the binaries attached to the GitHub release. Each carries a
PKGBUILD, a generated .SRCINFO, an ignore-everything .gitignore, and the
0BSD license the AUR expects for package sources.

update.sh bumps the release-based packages and regenerates the .SRCINFO
files; push.sh publishes each package to its AUR remote. The README now
points users at the AUR packages and keeps the in-tree PKGBUILD as the
development build.

packaging/aur/update.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+# Point the AUR packages at a new upstream release: rewrite pkgver, reset
+# pkgrel, refresh the checksums that depend on the version, and regenerate
+# every .SRCINFO.
+#
+# Usage: ./update.sh 1.12.0
+set -euo pipefail
+
+version="${1:-}"
+if [[ -z $version ]]; then
+  echo "usage: ${0##*/} <version>" >&2
+  exit 2
+fi
+
+cd "$(dirname "$(readlink -f "$0")")"
+
+# The -git package derives its pkgver from git describe, so only the two
+# release-based packages are versioned here.
+for pkg in proton-drive-for-linux proton-drive-for-linux-bin; do
+  sed -i -e "s/^pkgver=.*/pkgver=$version/" -e "s/^pkgrel=.*/pkgrel=1/" "$pkg/PKGBUILD"
+  (cd "$pkg" && updpkgsums)
+done
+
+for pkg in proton-drive-for-linux proton-drive-for-linux-git proton-drive-for-linux-bin; do
+  (cd "$pkg" && makepkg --printsrcinfo > .SRCINFO)
+  echo "updated $pkg"
+done