feat: "Gaming on NVIDIA" replaces the SteamOS conversion on NVIDIA PCs

gamescope's gaming mode shows a corrupted picture on NVIDIA (NVIDIA bug 5240452), also nested in KDE and in a KWin-only
session. With an NVIDIA GPU the conversion (gaming, boot, single, glyphs) is hidden unless already on, and nvidia takes its
place: Steam installed when missing and started at login on the Plasma desktop (user unit), with the sub-option bigpicture
for Big Picture or the normal window. The kernel parameters and early NVIDIA modules (RTX 20 series or newer) are kept as
part of it. tests/nvidia-test.sh covers the component and the menu rules against a fake GPU.

README.md

@@ -23,6 +23,12 @@ Steam Machine.
      as OS Version (e.g. `steamos-3.9`), `steam-machine` as OS Codename and
      Steamify's version as OS Variant. The OS name stays CachyOS
      ([details](TECHNICAL.md#what-steams-system-settings-show)).
+   - **NVIDIA graphics cards:** gamescope's gaming mode shows a corrupted
+     picture there (an open NVIDIA driver bug), so the conversion isn't
+     offered. **Gaming on NVIDIA** takes its place: Steam on the desktop,
+     installed if it's missing and started at login, in **Big Picture** or its
+     normal window as you choose
+     ([details](TECHNICAL.md#gaming-on-nvidia)).
 2. **SteamOS theme** - the Vapor look of SteamOS for the desktop, with
    **Add to Steam** in right-click menus
    ([details](TECHNICAL.md#steamos-desktop-look)).