HelpScreenshotImageProvider

Runtime scene-screenshot provider for the help browser.

Detailed description

Resolves `screenshot:<scene-id>?callout=<a>,<b>&mode=<m>&size=<wxh>&dpr=<n>` URLs into rendered QImages. Twin of HelpWidgetImageProvider — same two-tier cache (memory + disk under CacheLocation/octomy/help-scenes/<app-version>/), same placeholder-on-failure contract, same re-entrance guard.

The actual scene render is delegated through an injected RenderFn so libhelp stays free of libcatalog dependency. HelpActivity wires a concrete renderer that calls SceneScreenshotService::capture(...).

Cache key tuple is `<scene-id>|<mode>|<callouts>|<wxh>|<dpr>`. The callouts field is the comma-joined ordered list so different overlay orderings cache separately.

See also HelpWidgetImageProvider, SceneScreenshotService, SceneRegistry.