feat(info): show the install reason and drop the indentation

N? and `info PKG` now print three lines, none indented:

    PKG: <description>
    reason: explicitly installed | installed as a dependency | unknown | not installed
    required by: <installed dependents, comma separated> | nothing

The reason comes from a new <be>_reason backend function: apt-mark
showauto / showmanual (dpkg decides for a package in neither list), the
Install Reason of pacman -Qi, dnf repoquery --installed %{reason} (dnf4
and dnf5 words) and the Installed line of zypper info (rpm plus libzypp's
AutoInstalled when zypper is locked). A row whose old version is "-" is
not installed and the backend is not asked. The "(installed)" qualifier
moves to --help, README and DESIGN.

The rpm E2E fixture and e2e_reset now also mark su-dummy-a and su-dummy-c
as user-installed on dnf, which records plain rpm installs as "unknown",
so 1? and 2? show both reasons on all 12 distros. The verified commands
are recorded in discovery/{apt,pacman,rpm}.md.

tests/e2e/fixtures/rpm.sh

@@ -31,7 +31,10 @@
 #   so it is a pre-existing unneeded package (dnf repoquery --unneeded / zypper packages --unneeded).
 #
 # Optional (SU_FIXTURE_B_AUTO=1): su-dummy-b counts as installed as a dependency (dnf: `dnf mark
-#   remove`, zypper: /var/lib/zypp/AutoInstalled), like the pacman fixture's --asdeps.
+#   remove`, zypper: /var/lib/zypp/AutoInstalled), like the pacman fixture's --asdeps; on dnf
+#   su-dummy-a and su-dummy-c also count as installed by the user (`dnf mark install`): dnf4 reports
+#   packages installed with plain rpm as reason "unknown" (dnf5: "External User"). zypper needs
+#   nothing for them: a package that is not in AutoInstalled is user-installed.
 #
 # Re-runnable: re-running resets everything to the "1.0 installed, 2.0 available" state.
 #
@@ -198,7 +201,11 @@ fi
 if [ "${SU_FIXTURE_B_AUTO:-0}" = 1 ]; then
   case $BACKEND in
     zypper) grep -qx su-dummy-b /var/lib/zypp/AutoInstalled 2>/dev/null || echo su-dummy-b >>/var/lib/zypp/AutoInstalled ;;
-    dnf|yum) "$BACKEND" -q mark remove su-dummy-b >/dev/null ;;
+    dnf|yum)
+      # dnf4 spelling first, then dnf5's (which asks for confirmation)
+      "$BACKEND" -q mark remove su-dummy-b >/dev/null 2>&1 || "$BACKEND" -q -y mark dependency su-dummy-b >/dev/null
+      "$BACKEND" -q mark install su-dummy-a su-dummy-c >/dev/null 2>&1 || "$BACKEND" -q -y mark user su-dummy-a su-dummy-c >/dev/null
+      ;;
   esac
   log "su-dummy-b marked as installed as a dependency"
 fi