Normalize integer scalar keepbits in BitRound

[sapunyangkut]

Summary

• normalize integer-like keepbits values with operator.index before BitRound arithmetic
• add regression coverage for built-in, NumPy int32, and NumPy int64 scalars plus invalid inputs
• document the fix in the unreleased notes

Fixes #793.

Validation

• tests/test_bitround.py: 16 passed
• adjacent Quantize and FixedScaleOffset encode/decode tests: 2 passed
• numeric oracle: built-in/NumPy integer inputs remain finite, serialize as Python int, negative NumPy integers raise ValueError, and floats raise TypeError
• python -m compileall -q src/numcodecs/bitround.py tests/test_bitround.py
• git diff --check

The focused tests loaded the repository's Python modules directly because this Windows environment does not have the project's compiled C extensions and build toolchain. The full suite, documentation build, and coverage run were not executed locally; this remains a Draft while CI and maintainer review provide those gates.

Checklist

• Unit tests added for the changed behavior
• Affected tests pass locally
• Change documented in docs/release.md
• Full test suite and coverage pass in GitHub Actions
• Documentation builds in CI

AI assistance disclosure

This patch, tests, and pull request text were developed and checked with OpenAI Codex assistance. No independent human self-review is claimed; maintainer review is requested before merge.