Describe the application once, in a file every channel installs

A desktop entry gives a software centre a name and a one-line Comment, and
nothing else — no description, no licence, no age rating, no statement of what
hardware the interface was laid out for. GNOME Software and Discover both show
an application with no metainfo as an unexplained icon, and both packages this
repository produces were in that position.

packaging/paris.tourolle.darkroom.metainfo.xml is the AppStream component, and
it is installed by the PKGBUILD as well as by the Flatpak manifest because the
description, the licence fields and the OARS rating are facts about the
application rather than about how it was packaged. Writing it twice is how the
two packages start disagreeing.

Two things in it are easy to get wrong and are commented in place. The two
licence fields differ on purpose: metadata_license covers the file itself and
has to permit the unconditional redistribution and reformatting a catalogue
does, which GPLv3 does not, so it is CC0-1.0; project_license is the
application's own and reads GPL-3.0-or-later to match D8. And the component id
is not a fifth name but the same string as the desktop basename, the Flatpak
application id, and the app_id dr_ui::run sets — a rename that misses one costs
the icon or the association, and neither failure announces itself.

D15's decision that the target devices are a tablet and a desktop is stated as
a display_length requirement rather than left implicit, so a software centre
does not offer this on hardware where the photograph and the parameter panel
cannot both be on screen.

Validates clean under appstream-util validate-relax; appstreamcli --pedantic
reports only that the gitea URLs are unreachable from a machine that cannot
see that host.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packaging/PKGBUILD

@@ -37,6 +37,14 @@ package() {
     install -Dm644 "packaging/paris.tourolle.darkroom.desktop" \
         "${pkgdir}/usr/share/applications/paris.tourolle.darkroom.desktop"
 
+    # The same AppStream file the Flatpak installs, so a software centre
+    # describes the two packages identically instead of falling back to the
+    # desktop entry's one-line Comment for this one. Installed here rather than
+    # written twice: the description, the licence fields and the OARS rating
+    # are facts about the application, not about how it was packaged.
+    install -Dm644 "packaging/paris.tourolle.darkroom.metainfo.xml" \
+        "${pkgdir}/usr/share/metainfo/paris.tourolle.darkroom.metainfo.xml"
+
     # The icon's *name* is the contract, not its path: the desktop entry says
     # `Icon=paris.tourolle.darkroom` and the compositor resolves that through
     # the hicolor theme. Installed under 256x256 because that is the source's