Large test-bundle uploads complete reliably on multi-GB artifacts

Uploading large selective-testing bundles no longer runs out of memory. The multipart upload now keeps at most 10 parts in flight at a time and applies backpressure on the file read loop, so peak memory per upload stays bounded regardless of total bundle size, fixing the OOM that was killing multi-GB uploads. Archiving also happens in place directly from the products directory instead of copying each multi-hundred-MB test bundle into a staging temp directory first, removing tens of GB of redundant disk I/O on builds with hundreds of modules.