re-add XXH64 to read borg 1.x integrity data, #9935

[ThomasWaldmann]

What

Re-adds XXH64 support (removed in #9672 / #9750) so borg transfer can verify the integrity data of a borg 1.x source repository.

borg 1.x wrote XXH64 checksums into the repo index.N / hints.N integrity data. When #9672 switched borg 2.x file integrity to SHA256 and #9750 dropped the xxhash dependency, XXH64 disappeared from SUPPORTED_ALGORITHMS, so reading a legacy repo's integrity data now hits the "Unknown algorithm" path and skips verification. Fixes #9935.

How

Instead of re-introducing the external xxhash PyPI package (and the libxxhash msys2 system dependency that #9750 deliberately removed), this adds a small, dependency-free, streaming XXH64 implementation in Cython to borg.crypto.low_level:

• cdef class XXH64 (+ a one-shot xxh64() helper) with an interface compatible with the old xxhash.xxh64 FACTORY (XXH64() → .update() → .digest()/.hexdigest()), canonical big-endian digest.
• primes declared as real uint64_t C constants; byte-wise little-endian reads (correct on big-endian hosts too).
• XXH64FileHashingWrapper re-added and re-registered in SUPPORTED_ALGORITHMS.

XXH64 is only used on the read path — borg 2.x native repos keep writing SHA256. It is non-cryptographic and must not be used as a security mechanism (noted in the code).

Since we implement locally, there is intentionally no change to pyproject.toml or scripts/msys2-install-deps — nothing to re-add.

Tests

• Official xxHash sanity-check vectors: the test buffer generator is transcribed verbatim from xxHash tests/sanity_test.c; expected digests cross-checked against the reference implementation (empty = ef46db3751d8e999, empty+prime-seed = ac75fda2929b17ef, …). Covers empty / 1-byte / sub-32 / single-stripe / multi-stripe / 4 KiB, seeds 0 and PRIME32, streaming vs one-shot across every chunk alignment, and buffer-protocol inputs.
• Wrapper tests + a legacy XXH64 integrity round-trip through IntegrityCheckedFile (verify OK + corruption detected).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.73%. Comparing base (a50eb6f) to head (15eb490).
⚠️ Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9970      +/-   ##
==========================================
+ Coverage   85.70%   85.73%   +0.02%     
==========================================
  Files          95       95              
  Lines       16815    16856      +41     
  Branches     2577     2582       +5     
==========================================
+ Hits        14411    14451      +40     
- Misses       1668     1669       +1     
  Partials      736      736              

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