fix: don't drop colliding keys from ForestCAR

[lemmih]

Summary of changes

Changes introduced in this pull request:

• When gathering the mapping from hash to zstd frame offset, use a vector rather than a hashmap. Colliding hashes should be able to point to different zstd frames.

There is a small chance some of the archival ForestCAR files are invalid. Fortunately, it is easy to verify. I'll re-scan the files and re-generate any broken archives.

Reference issue to close (if applicable)

Closes

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.