Fix concurrent add dropping downloaded version

[KristofferC]

This has gotten little review of my own but putting it up at @IanButterworth request.

Summary

• make the post-download juliaup add conflict check version-aware
• commit and register the downloaded version when a concurrent system channel points at a different version, then move the channel to the requested version
• discard true duplicate downloads while preserving concurrent linked, direct-download, and alias channel name claims
• add regression tests for each conflict policy and document the updated locking flow

This fixes the race introduced by #1517 where add could print Installing ... followed by already installed, delete the completed temporary install, and leave the channel on an older version.

[IanButterworth]

Thanks