SteamOS NVIDIA Installer 0.1.8

Four fixes, and one change that decides what future releases are allowed to carry.
Installer Update delivers all of it, including the rebuilt overlay, so no new image
is needed.

• A release may now carry files this version does not know, instead of being
  refused whole. An installed tool judged a new release against the file list it
  was built with, so a manifest naming one unfamiliar file was rejected with
  Release has missing or unexpected files. Because the update window only ever
  offers the latest release, the first release to add a file would have left every
  older installation with no way forward, and the list could therefore never change.
  Unknown names and unknown fields are now ignored and named in a note, every signed
  file in the archive is still read and checksummed before anything is written, and
  only the files this version knows are installed. The format number stays strict, so
  a future change that must not be half applied can still refuse cleanly.

• The overlay model names no longer depend on an option that arrives by chance.
  Overlay levels 3 and 4 print the detected GPU and CPU above their readings only
  when MangoHud's mangoapp_steam option is present in the configuration Steam
  writes for the overlay. Steam writes that option itself, but not always: on the
  same machine, at level 4 with a game running, it was absent on one occasion with
  the names missing, and present later with the names shown. Level 4 also applies
  full, which clears the option unless a configuration line puts it back. So the
  feature worked for one person and not for the next, with no user action to explain
  it. Both labels are computed only in this project's overlay build, so requiring
  them to be non-empty is sufficient and is a condition this project controls. The
  rebuilt overlay differs from the previous one only by the 64 bytes of the removed
  condition.

• The Steam notification workaround now also applies when Steam starts from the
  desktop. It was attached to the Game Mode launcher alone, so after Steam verified
  its own files the opaque notification border came back and nothing reapplied the
  workaround until Game Mode was started again. Both launchers now pull it in, and it
  is reapplied whenever Steam restarts in either mode.

• The driver list stops presenting two different claims as one. Choosing a driver
  version offered ten releases, each marked with the same green indicator, which reads
  as an endorsement of ten drivers nobody here has run. NVIDIA documenting a GPU in a
  driver's range and this project having built and accepted that driver are separate
  claims and now look separate. The text above the list names the tested version, and
  when the tested one has aged out of the ten newest package sets the list says so
  rather than leaving the reader to work it out.

• Documentation. The documented way to switch the Remote Play receiver off named
  one launcher and therefore stopped working on the desktop in 0.1.7. It now names
  both and carries a command that writes both overrides. The encoder and notification
  sections say which launchers they attach to, and why the desktop has to have
  LIBVA_DRIVER_NAME removed while Game Mode never sets it.

Known and not fixed here. The notification workaround still selects the Steam client
asset by file name and hash, and this release cannot correct that: the signed bundle
does not carry that file, and a release that added it would have been refused by every
older installation. A later release can add it now that the change above is installed.
Receiving a stream in Desktop Mode still needs hardware decoding switched off in
Steam's own settings, unchanged from 0.1.7 and explained on the Troubleshooting page.

Under investigation and not affected by this release: on the tested PC, driven at
2560x1440 and 165 Hz over DisplayPort with HDR off, the Game Mode menus can draw with
visible glitches while a game runs and the performance overlay is off. It reproduces
with the overlay SteamOS packages and on an older integration, so it is neither new
here nor something this release changes. Two settings cleared it on that machine:
switching Automatic Scale Image off and stepping back one, or turning HDR on. The same
connection and mode is recorded as working with HDR on.

Validation. The published tree passes its suite on Linux, and ShellCheck is clean at
the severity the project's workflow uses. The corrected overlay patch was checked
against the exact MangoHud commit the builder uses, which is also the commit SteamOS
packages, and the overlay was rebuilt in a disposable SteamOS build root from the
published source, with its hashes compared in the build guest, in the build container
and on the host before the files were accepted. The documented receiver override
command was extracted from the manual and executed. The systemd directive that
reapplies the notification workaround was verified against systemd 257 on the tested
PC, with a deliberately invalid directive as a control. Hardware acceptance of the
overlay names follows this update on the tested PC.