Reliable uploads for large selective-testing bundles

Large artifact uploads no longer OOM. Multipart uploads now keep at most 10 parts in flight and apply backpressure on the read loop, so peak memory per upload stays bounded (10 × part size) regardless of bundle size. The bundle split introduced in this release shrinks the largest object, but this fix is what makes hundreds-of-modules, multi-GB builds upload reliably end to end, instead of only the smaller ones succeeding.