feat(show): merge the installed record with the upstream candidate

N?? and `show PKG` now print the installed record in the package
manager's own key/value layout, after one line explaining the format:

    # lines starting with > come from the upstream candidate: a differing
    value, or a field the installed record lacks

After each field whose candidate value differs comes the candidate's
line prefixed with "> "; fields only the candidate has are appended the
same way; installed-only fields stay; identical fields print once.
Multi-line values (pacman Optional Deps, dnf Description, RFC-822
continuation lines) are compared as blocks and a differing candidate
block gets "> " on every line. A package that is not installed shows the
candidate alone ("# not installed: this is the upstream candidate").

Merged where the backend gives comparable records for both sides:
- pacman: -Qi vs -Si REPO/PKG, REPO = the first repository -Sl lists
- apt: dpkg-query -s (Status, Conffiles, the long description; richer
  than apt-cache show PKG=<installed>, which gives the archive record
  when that version is still listed) vs apt-cache show --no-all-versions;
  Description-xx pairs with Description and a one-line candidate
  Description equal to the installed first line counts as identical
  (the archives ship the long text in Translation files)
- dnf: info --installed vs info --upgrades (--available lists other
  architectures first and nothing for an up-to-date package); not
  installed: --available; dnf5's differently padded keys are re-padded
zypper keeps `zypper info --requires`: rpm -qi and zypper info share
almost no keys and zypper cannot print the installed record itself.
No candidate, or not exactly one record per side, keeps the old output.

Formats verified in containers and recorded in discovery/*.md. Unit
tests replay recorded pacman, apt, dnf4 and dnf5 records; the E2E suite
checks 2?? on su-dummy-b on all 12 distros.

tests/unit/data/apt/show/perl-base

@@ -0,0 +1,21 @@
+Package: perl-base
+Source: perl
+Version: 5.36.0-7+deb12u4
+Installed-Size: 7643
+Maintainer: Niko Tyni <ntyni@debian.org>
+Architecture: amd64
+Replaces: libfile-path-perl (<< 2.18), libfile-temp-perl (<< 0.2311), libio-socket-ip-perl (<< 0.41), libscalar-list-utils-perl (<< 1:1.62), libsocket-perl (<< 2.033), libxsloader-perl (<< 0.31), perl (<< 5.10.1-12), perl-modules (<< 5.20.1-3)
+Provides: libfile-path-perl (= 2.18), libfile-temp-perl (= 0.2311), libio-socket-ip-perl (= 0.41), libscalar-list-utils-perl (= 1:1.62), libsocket-perl (= 2.033), libxsloader-perl (= 0.31), perlapi-5.36.0
+Suggests: perl, sensible-utils
+Conflicts: defoma (<< 0.11.12), doc-base (<< 0.10.3), mono-gac (<< 2.10.8.1-3), safe-rm (<< 0.8), update-inetd (<< 4.41)
+Breaks: amanda-common (<< 1:3.3.9-2), backuppc (<< 4.4.0-7~), bucardo (<< 5.5.0-1.1), debconf (<< 1.5.61), dh-haskell (<< 0.3), duck (<< 0.14.1), intltool (<< 0.51.0-4), kio-perldoc (<< 20.04.1-1), latexml (<< 0.8.4-2), libdevel-mat-dumper-perl (<< 0.42-3), libencode-arabic-perl (<< 14.2-2), libexception-class-perl (<< 1.42), libfile-path-perl (<< 2.18), libfile-spec-perl (<< 3.8400), libfile-temp-perl (<< 0.2311), libio-socket-ip-perl (<< 0.41), libmp3-tag-perl (<< 1.13-1.2), libsbuild-perl (<< 0.67.0-1), libscalar-list-utils-perl (<< 1:1.62), libsocket-perl (<< 2.033), libxsloader-perl (<< 0.31), mailagent (<< 1:3.1-81-2), perl (<< 5.36.0~), perl-modules (<< 5.36.0~), pod2pdf (<< 0.42-5.1), slic3r (<< 1.2.9+dfsg-6.1), slic3r-prusa (<< 1.37.0+dfsg-1.1), texinfo (<< 6.1.0.dfsg.1-8)
+Description: minimal Perl system
+Description-md5: 6f98ca50727514fe6b19872d5ede03ec
+Homepage: http://dev.perl.org/perl5/
+Essential: yes
+Pre-Depends: libc6 (>= 2.35), libcrypt1 (>= 1:4.1.0), dpkg (>= 1.17.17)
+Section: perl
+Priority: required
+Filename: pool/updates/main/p/perl/perl-base_5.36.0-7+deb12u4_amd64.deb
+Size: 1609596
+SHA256: d7d1943aec9597629bf73075efcc5ef6dc9bda96d78e80d843156ecd448478b8

tests/unit/data/apt/show/zsh

@@ -0,0 +1,21 @@
+Package: zsh
+Source: zsh (5.9-4)
+Version: 5.9-4+b15
+Installed-Size: 2461
+Maintainer: Debian Zsh Maintainers <pkg-zsh-devel@lists.alioth.debian.org>
+Architecture: amd64
+Depends: zsh-common (= 5.9-4), libc6 (>= 2.34), libcap2 (>= 1:2.10), libtinfo6 (>= 6), debianutils (>= 5.3-1~)
+Recommends: libc6 (>= 2.35), libgdbm6 (>= 1.16), libncursesw6 (>= 6), libpcre3
+Suggests: zsh-doc
+Description: shell with lots of features
+Description-md5: a129d6b2d23d2d5d3a6b822d3f8f856d
+Homepage: https://www.zsh.org/
+Tag: devel::interpreter, implemented-in::c, interface::shell,
+ network::client, protocol::ftp, role::program, scope::utility
+Section: shells
+Priority: optional
+Filename: pool/main/z/zsh/zsh_5.9-4+b15_amd64.deb
+Size: 907512
+MD5sum: 831903f59bf1ed613a5beeef9fc787ad
+SHA256: 8362cc8bbb530d393950522dc8a9df5a860a61227d62ff669a7524056e6289a8
+