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.

discovery/pacman.md

@@ -568,3 +568,20 @@ pman bugs (verified; relevant if the tool reuses `/etc/pacman.d/ignorepkg`):
   `pacman.conf` actually includes the file in `[options]` (silently ineffective, section 6).
 * The tool should read the effective list with `pacman-conf IgnorePkg` and write with anchored matches
   (`grep -qxF "IgnorePkg = $pkg"`, `sed -i "/^IgnorePkg = ${pkg}\$/d"` after escaping).
+
+## 13. Install reason (`N?` / `info`): `Install Reason` of `pacman -Qi`
+
+Verified 2026-10-10 on archlinux:latest (pacman 7.1.0), fixture `FIXTURE_ISOLATE=1 FIXTURE_ORPHAN=1 FIXTURE_SYNC=1`, `COLUMNS=0 LC_ALL=C pacman -Qi -- PKG`:
+
+```
+su-dummy-a       Install Reason  : Explicitly installed                              rc 0
+su-dummy-b       Install Reason  : Installed as a dependency for another package     rc 0   (-D --asdeps)
+su-dummy-c       Install Reason  : Explicitly installed                              rc 0
+su-dummy-orphan  Install Reason  : Installed as a dependency for another package     rc 0   (-U --asdeps)
+no-such-pkg      error: package 'no-such-pkg' was not found                          rc 1   (also for a package only in a sync db)
+```
+
+* `pacman -D --asexplicit su-dummy-b` / `--asdeps` flip the line at once. Works as `nobody` and with `db.lck` present (-Qi takes no lock). One line, no wrapping with `COLUMNS=0`.
+* `pacman -Qi sh` answers for the provider (`bash`), like the description and `Required By` already do.
+* Mapping: `Explicitly installed` -> explicit, `Installed as a dependency for another package` -> dependency, -Qi failing -> not installed, anything else (or no such line) -> unknown.
+* Fixture: `su-dummy-b` is installed `--asdeps`, `a`/`c` explicitly; `e2e_reset` re-applies both after its `pacman -U`. No change needed.