Export BuildFileIndex; decouple crawl from progress bar

[SUSTAPLE117]

Extract collector.buildFileIndex into an exported free function collector.BuildFileIndex

Progress is abstracted behind a new pkg/progress.Reporter interface (Start/Update/Done) with a NoOp default, replacing the hardcoded progressbar. BuildFileIndex depends only on the interface

[SUSTAPLE117]

Heads-up: the boostfree-endpoint consumer has pivoted to owning its scanning engine (its own group-scoped walker populating a fileindex.FileIndex), so it no longer calls collector.BuildFileIndex. This export is now unused by the endpoint.

Not asking to drop it — it's still useful for bagel's own CLI and the progress.Reporter abstraction is nice to keep. Just flagging that the endpoint dependency that motivated it is gone, so this PR can be evaluated purely on bagel-CLI merits (or closed if the CLI doesn't need it). bagel #64 and #65 remain consumed by the endpoint.