test: add historical index persistence compatibility coverage

[gouyt13]

Summary

Add regression coverage to ensure current readers can load and search indexes produced by v0.5.2. Freeze 12 index/reference pairs covering IVF quantized/raw storage, HNSW, and SymphonyQG raw/4-bit/8-bit storage under L2 and inner product, with the historical 128-dimensional padding for 65-dimensional inputs and persisted removal markers where supported.

C++ and Python tests check reference query results, resave/load migration, and malformed-file handling. Record the historical writer commit, generator, platform, and SHA-256 manifest so fixtures remain independent of the current writer. Document the coverage and regeneration procedure.

Only tests, test configuration, fixtures, and documentation change. Public APIs, production save/load implementations, persisted formats, and dependencies remain unchanged. Coverage does not imply that old readers can load new formats; historical IVF HNSW routing at 20,000 clusters is outside these small fixtures.

Testing

Validated the exact file contents committed in cd0cca6 on Linux x86-64 with GCC 14.3, Python 3.13, and RABITQ_ENABLE_NATIVE_OPTIMIZATION=OFF:

• Historical C++ tests: 12 passed.
• Full C++ suite: 332 passed, 1 conditional skip; the independent OpenMPBinding test passed.
• Historical Python tests: 20 passed.
• Full Python suite using the rebuilt extension and rebuilt C++ examples: 988 passed, 2 platform-specific repaired-wheel checks skipped.
• All 24 frozen fixture/reference hashes match the manifest.
• Full core/binding and historical-test clang-tidy, C++ formatting, Python formatting/lint, strict MkDocs build, and git diff --check: passed.

Windows, macOS, and native ARM64 were not executed locally and remain for CI validation.