Cut the v0.6.0 release

Summary

Cut the v0.6.0 release once every other issue in the milestone has merged. This is the last item in
the milestone and it is blocked by all of them.

There are 161 commits since the v0.5.6 tag, and pyproject.toml:12 still reads
version = "0.5.6".

Scope

1. Bump the version. pyproject.toml:12 is the whole change. polyfile/polyfile.py:19 reads
   __version__ from package metadata through importlib.metadata.version("polyfile"), so nothing
   else carries a hard-coded version string.

2. Write polyfile_0.6.0.md. Follow the structure of polyfile_0.5.6.md: Performance
   Improvements, Bug Fixes, New Features, Magic Definitions, Breaking Changes, Dependencies. Omit a
   section that has nothing in it.

   The release covers two bodies of work. Summarize rather than list every issue:

   • libmagic fidelity. The definitions moved to 5.48, and all 88 stems of the upstream corpus
     now match what file reports. tests/test_magic.py::KNOWN_FAILURES is empty. #3480 records how
     the previous fourteen failures were retired.
   • Packaging. setup.py was replaced by pyproject.toml and the in-tree PEP 517 backend
     build_backend.py (#3513).

   Call out under Breaking Changes anything in this milestone that changes observable output. At
   minimum: #3499 changes how an untyped indirect offset reads its field, #3500 changes what a
   BOM-prefixed JSON document reports, #3497 changes the accepted values of --format, and #3465
   removes the try_all_offsets parameter from Matcher and Analyzer.

3. Verify the distribution. uv build, then uvx twine check dist/*. The distributions job in
   .github/workflows/tests.yml already asserts that the wheel is built from the sdist, that no
   non-permissive .ksy reached the sdist, and that the package-data counts match git ls-files;
   confirm it is green rather than re-deriving it.

Note

Publishing is out of scope for the pull request. .github/workflows/pythonpublish.yml runs on
release: published and uploads through PyPI Trusted Publishing, so the actual release is cut from
the GitHub UI after this merges.

Wait for the other milestone issues to merge, then update version = "0.5.6" in pyproject.toml:12 and write polyfile_0.6.0.md following polyfile_0.5.6.md. Summarize the listed libmagic and packaging work, document observable breaking changes, run uv build and uvx twine check dist/*, and confirm the distributions job in .github/workflows/tests.yml is green.