refactor: migrate packaging from setup.py+versioneer to pyproject.toml+setuptools-scm

[alubbock]

Summary

• Replaces setup.py, setup.cfg, MANIFEST.in, and versioneer.py with a single pyproject.toml
• Uses setuptools-scm for version management (derives version from git tags, writes microbench/_version_scm.py at install time — build dependency only, not runtime)
• __version__ in __init__.py now reads from _version_scm.py with fallback to importlib.metadata (stdlib), then 'unknown' — no new runtime dependencies
• Adds microbench/_version_scm.py and dist/ to .gitignore
• Updates CI release job to use python -m build instead of deprecated python setup.py sdist
• Adds setuptools-scm and pip install -e . to the CI test job so the version file is available during tests

Test plan

• Verify pip install -e . works from a clean checkout
• Verify python -m build produces a valid sdist and wheel
• Verify import microbench; microbench.__version__ returns the correct version