QuantStudio integrated historical downloader

Based on the validated standalone v4.1 downloader. Decoder, source configuration,
UnRAR and bundled VIX/master snapshots are unchanged. The app uses its own Python
runtime, so a second runtime or downloader executable is not required.

Integration additions: exact --dataset-root, per-job identity in progress output,
durable safe-Stop file plus existing stdin Stop, and authenticated app controls.
The directory selected in the app directly contains ticks, _state and the catalog.
No additional dataset directory is appended in the integrated workflow.

Downloads need access to the configured data provider. 401/403 terminates the
run without trying alternate access or credentials. VIX is a bundled historical
snapshot or the owner's D99 export, not an anonymous live VIX data service.
UnRAR license is in bin. Preserve _state with the dataset to resume or rebuild
the catalog. Existing catalogs are merged, never replaced with only this run.
