Updates: offer an RPM only to a copy rpm owns, and never read an unknown version as older

On a distribution without rpm, a locally built package that mirrors the spec puts the nightly tree
at /opt/DodoSSH.Nightly, and that path was the whole test for "the RPM installed this". The channel
then asked rpm for the installed version, got nothing because there is no rpm, read the silence as
"0", and offered every nightly as newer, fetching a 56 MB RPM into the profile directory that the
machine could never install, every six hours.

UpdateChannels.LinuxChannel still checks the path first, then asks rpm through the new
LinuxUpdateChannel.ForInstall. A copy rpm has no record of gets an UnavailableUpdateChannel that
says it was installed outside the dodossh-nightly (or dodossh) RPM and should be updated with the
package manager that installed it. UnavailableUpdateChannel gains an optional Reason for that,
UpdateViewModel exposes it as UnsupportedReason, and the desktop preferences page shows it in place
of the sentence about checkouts and unzipped copies. That costs one rpm launch at start, and only
where the path already matched.

CheckAsync no longer falls back to "0". If rpm stops answering after composition, the check throws
instead, so CHECK NOW reports it and the timer stays quiet, and nothing is offered or fetched. The
comment that called the fallback a deliberate choice is replaced with the reason it was wrong.

LinuxUpdateChannelTests: an unknown installed version throws and downloads nothing (replacing the
test that asserted the old behaviour), a copy rpm does not own is unavailable and says why, and a
copy rpm owns gets the channel. The newer, equal and older comparisons were already covered.
UpdateFlowTests covers the reason reaching the view model. Manual check 19.13.

App tests 744/752 with 8 skips, layout tests 202/203 with 1 skip; the full solution passes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/manual-checks.md

@@ -3538,3 +3538,16 @@ Register a device, sync something, then `sudo dnf remove dodossh`.
 make an uninstall delete a user's un-synced work — the Linux spelling of the mistake ADR 0013 decision 2
 exists to prevent. Files left behind under `/opt/DodoSSH` are files the package did not own, which means
 something wrote into the install directory at runtime and would survive a reinstall.
+
+### 19.13 A copy at the same path that rpm does not own is offered nothing
+
+On a distribution without rpm — Arch, say, with a locally built package that puts the nightly tree at
+`/opt/DodoSSH.Nightly` — open PREFERENCES → UPDATES.
+
+**Pass:** **CHECK NOW** is disabled and the row says this copy was installed outside the `dodossh-nightly`
+RPM and should be updated with the package manager that installed it. `~/.local/share/dodossh.nightly/updates/`
+stays empty however long it runs.
+
+**Failure means:** a banner offering a nightly is the path being taken as proof the RPM is installed, or an
+rpm that cannot answer being read as an old version — the fault this check was written for, which offered
+every nightly as newer and fetched a 56 MB package the machine could never install, every six hours.