[gouyt13]

Summary

Add one-line __repr__ summaries for IVF, HNSW and SymphonyQG indexes, showing configuration, counts and build state. Read metadata under one existing access guard; concurrent reads remain allowed and active mutations retain the busy error contract. Update stubs and usage documentation.

Testing

• Seven new cases cover unbuilt/built/loaded states, raw/quantized storage, count changes and capacity growth. Existing concurrency tests also exercise repr/str.
• Python suite: 811 passed, 6 skipped. Final installed-wheel checks: 55 passed; independent focused review: 29 passed.
• Portable wheel build/install, full clang-tidy, C++ formatting, Ruff, mypy/Pyright, strict MkDocs and git diff --check passed.

Skipped tests require C++ example executables (4) or non-host platform wheels (2). Windows/ARM execution remains for PR CI.