Add layout diagnosis: font metrics in the log, --snapshot option

On the Steam Frame, the fixed 1200x800 QML layout overflows: about the
bottom quarter is cut off and the navigation icons overlap their text.
Forcing 96 DPI and switching the Quick Controls style didn't help, so the
cause has to be measured rather than guessed.

- SetWidget() logs the application font (family, pt, px), what 20 pt
  (MyText) is in px, the screen's DPI and the device pixel ratio.
- --snapshot <file> writes the rendered overlay (RenderBackend::readback)
  as PNG at most once per second while it is visible. A time interval,
  because OVRAS only renders when the scene changes. Written to a
  temporary file and renamed, so readers never see a partial file.
- run.sh passes arguments after the graphics API to the program.

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

docs/steamframe/README.md

@@ -124,11 +124,12 @@ docker run --rm -v "$PWD:$PWD" -w "$PWD" ovras-steamframe-cross-arm64 \
 | Command | Effect |
 |---|---|
 | `./install.sh` | Registers with SteamVR (auto-start) and starts |
-| `./run.sh opengl\|vulkan` | Restarts with that graphics API |
+| `./run.sh opengl\|vulkan [options]` | Restarts with that graphics API; further options go to the program |
 | `./uninstall.sh` | Unregisters |
 | `AdvancedSettings.log` | Log (in the working directory, i.e. this folder) |
 
-Program options (new): `--graphics-api opengl|vulkan` (default `opengl`). Existing ones
+Program options (new): `--graphics-api opengl|vulkan` (default `opengl`); `--snapshot <file>`
+(diagnosis: writes the rendered overlay as PNG at most once per second while visible). Existing ones
 include `--force-install-manifest` and `--force-remove-manifest`; see `--help`.
 
 ## Steam Frame facts the code relies on