feat: packages and a signed repository for Debian and Fedora

The code works on those distributions now; there was still nothing to
install. This adds the two packages and, more to the point, somewhere for
them to live that hands out updates - a package a user has to notice a new
version of and download again is not much better than a checkout.

Both are built from `make install` and nothing else. A packaging script that
lists the installed files a second time is a second description of the
layout, and the two drift the first time a file moves; here the Makefile
stays the only place that says where anything goes. The .deb is staged and
wrapped with dpkg-deb, the .rpm goes through a spec whose %install is the
same make invocation. Both are architecture-independent, so one file each
covers Debian, Ubuntu and their derivatives on one side and Fedora, RHEL and
openSUSE on the other.

The version is not written down twice either. The Makefile has it, the
control file and the spec take it as a placeholder, and check-version.sh
refuses a tag that disagrees - otherwise a v1.0.4 release quietly ships a
program that reports 1.0.3.

The release workflow builds both in a Debian and a Fedora container, signs
the RPM where there is a native rpm-sign, attaches both to the GitHub
release, and then adds them to an APT and a DNF repository on gh-pages,
regenerating the indexes over every version ever published so that pinning
and going back to one still work. Missing the signing key is not an error:
it builds, it says in the log that the repositories were left alone, and the
packages are still on the release.

There is also a check workflow, which is the first time shellcheck actually
runs on this.

Neither package carries a maintainer script. The units are enabled per user
by the program itself, so there is nothing for a package to do as root - and
nothing it could do about the changes in a user's home either, which is why
both descriptions say to run `disable` before removing.

packaging/README.md has the two things that cannot be automated: making the
signing key, and pointing Pages at the branch.

packaging/deb/copyright

@@ -0,0 +1,20 @@
+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: middleclick-autoscroll
+Source: https://github.com/Felitendo/middleclick-autoscroll
+
+Files: *
+Copyright: 2026 Felitendo
+License: GPL-3+
+
+License: GPL-3+
+ This program is free software: you can redistribute it and/or modify it under
+ the terms of the GNU General Public License as published by the Free Software
+ Foundation, either version 3 of the License, or (at your option) any later
+ version.
+ .
+ This program is distributed in the hope that it will be useful, but WITHOUT ANY
+ WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
+ PARTICULAR PURPOSE. See the GNU General Public License for more details.
+ .
+ On Debian systems the full text of the GNU General Public License version 3 can
+ be found in /usr/share/common-licenses/GPL-3.