Investigate streaming upload pipeline #2886
Description
Eliminate wait times by overlapping the collection, processing, bundling, and upload phases. Currently these run sequentially (collect all → process all → bundle all → upload). Use a streaming pipeline where files start uploading as soon as the first bundle chunk is ready.
- Stage 1 (Producer): File collection thread
- Stage 2 (Processor): Sourcemap rewriting thread pool
- Stage 3 (Bundler): Bundle building thread
- Stage 4 (Uploader): Upload thread pool