archive: preserve 4GiB+ entry sizes on LLP64

[i80487]

Note: This PR was created as part of a test of GitHub Copilot agents. I believe the proposed changes are relevant and consistent, but I don’t have enough experience with the git-for-windows source code to evaluate them properly.

I was unable to verify that these changes work correctly using the test script ‎t/t5005-archive-large-files.sh, because it appears that the GIT_TEST_LONG=true option and the SIZE_T_IS_64BIT prerequisite must be enabled, and I don’t know how to configure this in GitHub CI/CD.

Feel free to modify, defer, or reject this PR.

Details:

git archive still carried unsigned long size plumbing in both zip and tar paths, which truncates 4GiB+ entry sizes on LLP64 Windows before Zip64/PAX logic can trigger. As a result, large entries could be archived with silently wrapped sizes even though the surrounding format support already handled 64-bit lengths.

• Common archive callback plumbing

  • Widen write_archive_entry_fn_t and the write_archive_entries() call chain from unsigned long to size_t.
  • Update indirect callers so large blob sizes are forwarded without narrowing before they reach the backend writers.

• Zip backend: make Zip64 decisions from real sizes

  • Convert ZIP entry size and compressed-size state to size_t.
  • Keep CRC as uint32_t; do not conflate checksum width with payload size width.
  • Clamp 32-bit header fields only at the final non-Zip64 write sites, after the Zip64 decision is made.
  • Add a chunked CRC helper so in-memory CRC computation remains correct for size_t-sized buffers.

• Tar backend: keep tar/PAX consistent with widened sizes

  • Update tar entry and extended-header size handling to size_t.
  • Preserve large regular-file sizes through PAX extended-header generation instead of narrowing them in the callback layer.
  • Adjust buffered tar write helpers to use size_t consistently.

• Regression coverage

  • Add a new expensive SIZE_T_IS_64BIT archive test that creates a real 4GiB file, commits it, archives it, and verifies the reported entry size in both zip and tar output.
  • Register the test in t/meson.build.