build-aux: Add an Arch package build

Builds federated-calendar from main for the "federated" pacman
repository in Gitea's package registry. The version comes from
meson.build plus the commit count and hash, so every commit
upgrades the package.

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

build-aux/arch/.gitignore

@@ -0,0 +1,5 @@
+/src/
+/pkg/
+/federated-calendar/
+*.pkg.tar.*
+*.log

build-aux/arch/PKGBUILD

@@ -0,0 +1,55 @@
+# Maintainer: David Young <david@federated.computer>
+
+pkgname=federated-calendar
+pkgver=52.alpha.r6135.ga818b79c
+pkgrel=1
+pkgdesc="Calendar for Federated Computer Cores, based on GNOME Calendar"
+arch=(x86_64)
+url="https://gitea.federated.computer/Federated_Computer/federated-calendar"
+license=(GPL-3.0-or-later)
+depends=(
+  dconf
+  evolution-data-server
+  fribidi
+  geoclue
+  glib2
+  glibc
+  graphene
+  gsettings-desktop-schemas
+  gtk4
+  hicolor-icon-theme
+  libadwaita
+  libedataserverui4
+  libgcc
+  libgweather-4
+  libical
+  libsoup3
+  pango
+)
+makedepends=(
+  git
+  glib2-devel
+  meson
+)
+source=("git+$url.git")
+b2sums=('SKIP')
+
+pkgver() {
+  cd $pkgname
+  local version
+  version=$(sed -n "s/^ *version: '\(.*\)',$/\1/p" meson.build)
+  printf "%s.r%s.g%s" "$version" "$(git rev-list --count HEAD)" "$(git rev-parse --short HEAD)"
+}
+
+build() {
+  arch-meson $pkgname build
+  meson compile -C build
+}
+
+check() {
+  meson test -C build --print-errorlogs
+}
+
+package() {
+  meson install -C build --destdir "$pkgdir"
+}