SteamOS NVIDIA Installer 0.1.9

This release exists so that a later one can replace Gamescope on a machine that is
already installed. Until now it could not: the signed bundle carried eighteen files
and Gamescope was not among them, so changing it meant building a new USB image and
installing from scratch. That is also the only reason the display fix described below
cannot arrive today.

• Add Gamescope to the set of files a release may deliver, as its own all or nothing
  group beside the performance overlay. Its binary, the provenance file that records
  which source commit it was built from, and its license now sit where the existing
  installer function already looks for them, so that function verifies the binary
  against its own checksum, refuses an artifact built from a commit it does not
  recognise, writes the session override and regenerates the addon manifest. No new
  installation code was needed.
• Carry a Gamescope build that fixes corrupted Game Mode menus on NVIDIA. With a
  menu open, Gamescope could hand the game and Steam's interface straight to two
  hardware planes and let the driver blend them, and that blend draws displaced
  fragments, black rectangles and coloured bands over much of the screen. The build
  refuses that route when a layer needing alpha blending would be placed on a plane,
  on a display device the kernel reports as nvidia-drm. Set
  GAMESCOPE_NVIDIA_COMPOSITE_ALPHA=0 in the Gamescope session to opt out of that
  one reason; every other reason to composite still applies.

An installation running 0.1.8 will not receive that Gamescope build from this
release, and will say so. An installed version only writes the files it was built
knowing about, and it names the ones it skipped. This release gives it the file list
that includes Gamescope, so the release after this one can deliver the binary. A
system installed from a new image gets it immediately.

Nothing else changes. The recovery base remains SteamOS 3.8.14.

Validation: the source tree passed its test suite, which gained four tests for the
changes above, 272 in total. The Gamescope change was verified on one PC with an
RTX 5060 before it was packaged, over DisplayPort and HDMI, at 2560x1440 in 165, 144,
120 and 60 Hz and at 1920x1080 in 120 and 60 Hz, with HDR off and on, with VRR off and
on where the connection offered it, and with the performance overlay off and on. The
menus were checked in the main Steam interface, over a running game, during a
controller notification and through repeated rapid opening and closing. Each result
was attributed from Gamescope's own record of which route it chose rather than from
appearance alone, and the corruption was deliberately reproduced with the change
disabled on both connections, so a clean picture is not simply a machine that stopped
failing. What is not established: the precise defect inside the closed driver is not
identified, this avoids a presentation route rather than repairing it, and it is not
evidence that every similar report elsewhere has the same cause. No performance claim
is made; power was sampled with the route changed but the scene was a game with moving
opponents rather than a repeatable benchmark.