Replace N per-vendor .cache files with a single system_presets.cache
that holds all vendors and presets in one serialized blob.
Cache load is now all-or-nothing: on hit all vendors are applied from
the bundle (sub-second); on miss all vendors are parsed from JSON and
a fresh bundle is written to the user cache dir.
Invalidation is driven by bundle_key - a sorted concatenation of all
vendor JSON version strings. Any vendor update invalidates the whole
cache and triggers re-parse on next launch.
Guide wizard (WebGuideDialog) loads the bundled cache into a plain
PresetBundle instead of a separate VendorGuideData struct, removing
the duplicate data model.
generate_system_cache simplified from a per-vendor loop to a single
save_system_presets_cache() call producing one output file.
