test: validate ZIP64 extraction with generated fixtures

[nextestudios]

Summary

• New Zip64Tests generate ZIP64 fixtures at test time in temp dirs (nothing large is committed):
  • a Deflate entry of 4 GiB + 1 MiB + 123 bytes (ZIP64 size fields) followed by a small entry: extracted through SafeExtractor with exact size, CRC-32 checked, samples compared around the 4 GiB boundary;
  • 70,000 entries (ZIP64 end-of-central-directory record, asserted in the file tail): all listed, entries beyond feat: support split and multi-volume archives #65,535 extracted by selection;
  • the MaxEntries limit still refuses a ZIP64 archive before anything is written.
• No code change was needed: SharpCompress 1.0 reads both cases correctly. docs/archive-support.md, fixture README and both changelogs record the observed result.

Tested in CI

• Windows runner: all unit tests pass; the whole unit suite (including the 4 GiB test) ran in ~41 s.

Not covered

• An archive that is itself larger than 4 GiB on disk (ZIP64 local header offsets) — documented as not tested.

Closes #63

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