test: add cross-language deletion vector fixtures

[wirybeaver]

Summary

Partially close #916

• add Java- and Go-generated single-blob and multi-blob Puffin deletion-vector fixtures
• add a Go fixture containing array, run, and bitmap Roaring containers across multiple high-32-bit keys
• verify Puffin metadata, manifest blob ranges, referenced data files, cardinality, and every decoded position
• document fixture provenance, pinned source revisions, regeneration steps, and checksums

This adds cross-language coverage beyond C++ self-roundtrips and follows up on the deletion-vector work discussed in #637 and #777.

Testing

• puffin_test (31/31 tests passed with GCC 14)
• regenerated the Go fixtures from Apache Iceberg Go commit 3020adbbc3faff047da6f483f739f1b5e1de611b and compared them byte-for-byte
• verified the all-container fixture contains one array, one run, and one bitmap container in each high-32-bit bucket

[wgtmac]

Thanks for working on these! Let me know when you think they are ready for review.

[wirybeaver]

@wgtmac It's my pleasure. Will have a self-review and open PRs by end of this weekend!

[manuzhang]

I don't think this should block other DV PRs and I think basic DV tests should be self-contained. For cross-language compatibility, we may make use of the newly created iceberg-verification project.

[wirybeaver]

@manuzhang Yeah, I also reconsidering to refactor the PR order of #916 to focus on delivering features first and conflate the validation PRs into the feature PRs. Thanks for pointing out the iceberg-verification project.

[wirybeaver]

Closing the standalone fixture PR as part of the four-feature consolidation in #916. Focused tests now accompany the delivered features; this fixture-only prerequisite is no longer part of the roadmap.