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/apt.md

@@ -605,3 +605,20 @@ Container start overhead is ~0.2-0.3 s; `docker pull` of the five images took 3-
 Everything else compared with `diff` over a 385-line transcript of ~60 commands (update, list, simulate, hold, show/policy/dpkg -s, depends/rdepends, real upgrade, dist-upgrade, purge, autoremove, error paths): identical wording and exit codes across the five images apart from the rows above and checksums/sizes. Specifically identical: `apt list` stderr warning, `Inst`/`Conf`/`Remv`/`Purg` formats, `kept back` block, `apt-mark` messages, `apt-cache depends/rdepends` formats, `E:`/`W:` texts and exit codes (0 / 100), lock message, conffile failure, Essential protection message.
 
 Pre-existing docker images were not touched (`images-before.tsv` compared: nothing missing or changed); only `debian:12`, `debian:13`, `ubuntu:22.04`, `ubuntu:24.04`, `ubuntu:latest` were pulled and only `docker run --rm` containers were used (one hung throwaway container of mine, caused by a test that re-ran the fixture over a locally modified conffile, was `docker kill`ed).
+
+## 13. Item 13 - install reason (`N?` / `info`): `apt-mark showauto` / `showmanual`
+
+Verified 2026-10-10 on debian:12 (apt 2.6.1), debian:13 (3.0.3), ubuntu:22.04 (2.4.14), ubuntu:24.04 (2.8.3) and ubuntu:latest (26.04.1, apt 3.2.0), fixture `SU_FIXTURE_OFFLINE=1 SU_FIXTURE_ORPHAN=1 SU_FIXTURE_B_AUTO=1`, `LC_ALL=C`. Identical on all five:
+
+```
+$ apt-mark showmanual su-dummy-a   -> su-dummy-a        (rc 0)   $ apt-mark showauto su-dummy-a   -> (empty, rc 0)
+$ apt-mark showmanual su-dummy-b   -> (empty, rc 0)              $ apt-mark showauto su-dummy-b   -> su-dummy-b   (rc 0)
+$ apt-mark showauto su-dummy-orphan -> su-dummy-orphan            $ apt-mark showmanual no-such-pkg -> (empty, rc 0)
+$ apt-mark showmanual su-dummy-a su-dummy-b su-dummy-c -> su-dummy-a     (only the named packages)
+```
+
+* Chosen: `apt-mark showauto PKG` -> dependency; else `apt-mark showmanual PKG` -> explicitly installed; in neither list -> `dpkg-query -W -f '${db:Status-Abbrev}' PKG`: installed (2nd character neither `n` nor `c`) counts as manual, else not installed. No image had an installed package in neither list (installed = manual + auto exactly: debian:12 92 = 2 + 90, ubuntu:24.04 96 = 93 + 3, ...), so that branch is a safety net.
+* Read-only: both work as `nobody` (`setpriv --reuid=65534`) and while another process holds dpkg's `lock-frontend` and `lock` (fcntl, like dpkg; checked with `apt-mark hold`, which fails then with `dpkg frontend lock was locked by another process with pid N`, exit 100): 0 s wait, same answers.
+* A removed package with its configuration files kept (`rc`) is in neither list: not installed. A package installed with `dpkg -i` is manual (also `su-dummy-c` reinstalled with `dpkg -i` after `dpkg -r`).
+* apt-mark prints a native package without its architecture qualifier (`apt-mark showauto libc6:amd64` -> `libc6`) and nothing for an arch that is not installed (`libc6:i386` -> empty); a name that is not a package is treated as a pattern (`apt-mark showmanual 'su-dummy-?'` and `'su-dummy-.'` -> `su-dummy-a`), exact names win (`libapt-pkg6.0`, `libbz2-1.0`, `libdb5.3` answered for themselves). Hence the tool matches the output line by line against `PKG` (or `PKG` without `:arch`).
+* Fixture: `su-dummy-a`/`-c` are manual (`dpkg -i`), `su-dummy-b` is auto with `SU_FIXTURE_B_AUTO=1` (`apt-mark auto su-dummy-b`); no change needed.

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.

discovery/rpm.md

@@ -704,3 +704,42 @@ Inside `proof.sh` (warm containers): forced refresh (`su_refresh`) 0.5-2.6 s (up
 * zypper exit codes 100-103, 106, 107 were not reproduced (see 7.2). `opensuse-slowroll`, MicroOS/Aeon (transactional), SLES were not tested.
 * Multi-arch (x86_64 + i686) and installonly kernels were not exercised; the parser keys on `name.arch` and keeps the highest installed `evr` per key for the "old" column.
 * Docker images pulled during this work (all kept, none removed or re-tagged): `almalinux:9`, `almalinux:10`, `rockylinux:9`, `rockylinux/rockylinux:9`, `rockylinux/rockylinux:10`, `quay.io/rockylinux/rockylinux:9`, `quay.io/rockylinux/rockylinux:10`, `opensuse/tumbleweed:latest`, `opensuse/leap:latest`, `fedora:latest`.
+
+## 14. Install reason (`N?` / `info`)
+
+Verified 2026-10-10, fixture `SU_FIXTURE_ISOLATE=1 SU_FIXTURE_ORPHAN=1`, `LC_ALL=C`.
+
+### 14.1 DNF: `dnf -q repoquery --installed --queryformat '%{reason}\n' PKG`
+
+Same answers on almalinux:9 and rockylinux/rockylinux:9 (dnf 4.14.0), almalinux:10 and rockylinux/rockylinux:10 (dnf 4.20.0); fedora:latest (Fedora 44, dnf5 5.4.3.0) uses other words:
+
+| state | dnf4 | dnf5 |
+|---|---|---|
+| installed with plain `rpm -Uvh` (fixture's a, b, c) | `unknown` | `External User` |
+| `su-dummy-orphan` (pulled in by `dnf install su-dummy-orphan-parent`) | `dependency` | `Dependency` |
+| `dnf install su-dummy-a` pulling in `su-dummy-b` | a `user`, b `dependency` | a `User`, b `Dependency` |
+| not installed (in a repo or unknown) | no output, rc 0 | no output, rc 0 |
+
+* `%{reason}` exists on every target, `--qf` and `--queryformat` both work. dnf4 ends the value with a newline, dnf5 does not (several installed versions would run together), so the format carries its own `\n` and the first non-empty line is used. Other values in the code: `weak-dependency`/`Weak Dependency` (dependency), `group`/`Group` (explicit: asked for with its group); dnf5 `External User` counts as user-installed (`repoquery --userinstalled` lists those packages) -> explicit; dnf4 `unknown` -> unknown.
+* Fallback `dnf history userinstalled` not needed and not used: dnf5 has no such command (`Unknown argument "userinstalled"`, rc 2), and on dnf4 it lists the fixture's rpm-installed a, b, c (reason `unknown`) as "Packages installed by user", i.e. it would call b explicit.
+* Marks: dnf4 `dnf mark remove PKG` -> `dependency`, `dnf mark install PKG` -> `user` (`mark user`/`mark dependency`: invalid choice, rc 2); dnf5 `dnf -y mark dependency|user PKG` (without `-y`: "Operation aborted", rc 1; `mark remove|install`: unknown argument, rc 2). A mark survives a later `dnf upgrade` and an `rpm -Uvh --oldpackage --replacepkgs` back to 1.0 (the e2e reset).
+* Works as `nobody` (setpriv / python3 on almalinux:10), with or without `-C`, in 110-150 ms (dnf4) / 45 ms (dnf5), and does not touch the repositories even with the distro repos re-enabled and their metadata expired.
+* Fixture consequence: on dnf the rpm-installed a and c would print `reason: unknown`; `SU_FIXTURE_B_AUTO=1` now also runs `dnf mark install su-dummy-a su-dummy-c` (dnf5: `-y mark user`), and `e2e_reset` re-marks a/c user and b dependency.
+
+### 14.2 ZYPPER: `Installed` line of `zypper --non-interactive --no-refresh --quiet info PKG`
+
+opensuse/tumbleweed (zypper 1.14.101, libzypp 17.38.16) and opensuse/leap 16.0 (same versions), identical, fixture with `SU_FIXTURE_B_AUTO=1`:
+
+```
+su-dummy-a              Installed      : Yes                    rc 0
+su-dummy-b              Installed      : Yes (automatically)    rc 0   (listed in /var/lib/zypp/AutoInstalled)
+su-dummy-orphan         Installed      : Yes (automatically)    rc 0   (pulled in by zypper install su-dummy-orphan-parent)
+su-dummy-orphan-parent  Installed      : No                     rc 0   (Status : not installed)
+bash                    Installed      : Yes                    rc 0   (Repository : @System, repos isolated)
+no-such-pkg             package 'no-such-pkg' not found.        rc 104
+```
+
+* The block also has `Installed Size : 15 B`: match `^Installed *:` only.
+* Cross-check: the flag is exactly membership in `/var/lib/zypp/AutoInstalled` (bare package names, plus `pattern:...` lines and `#` comments; 0644 root, world-readable). Deleting `su-dummy-b` from it turns the line into `Yes` immediately; an `rpm -Uvh --replacepkgs` reinstall keeps it; `zypper install su-dummy-a` (rpm -e'd first) installs b with `Yes (automatically)` and adds it to the file; naming b on the command line (`zypper update su-dummy-a su-dummy-b`) drops the flag (b becomes user-installed).
+* Works as `nobody`. As root, while another process owns the zypp lock (`/run/zypp.pid` with a live pid), `zypper info` exits 7 ("System management is locked by the application with pid N (sleep)") - as `nobody` it still answers. Fallback in that case (and for exit 104): `rpm -q PKG` fails -> not installed; else membership in AutoInstalled -> dependency / explicit; unreadable -> unknown.
+* Fixture: a/c (plain rpm, not in AutoInstalled) are `Yes`, b is `Yes (automatically)` with `SU_FIXTURE_B_AUTO=1`; `e2e_reset` re-adds b to AutoInstalled. No change needed.