Add pathlib.Path support for audio file I/O

[gijzelaerr]

Summary

Picks up @psobot's work from #451 to add pathlib.Path support throughout the audio file I/O API.

• Uses os.fspath() via pybind11 to convert path-like objects to strings (PathUtils.h)
• Adds py::object overloads in ReadableAudioFile, WriteableAudioFile, and AudioFile constructors that accept pathlib.Path (and any os.PathLike)
• Fixes write-mode overload resolution: properly dispatches between path-like and file-like objects instead of blindly calling pathToString()
• Updates type hints to accept Union[str, Path]

Changes from psobot's original PR

• Fixed build errors from the original branch
• Fixed type hint issues (pathLikeOrStr references)
• Added proper fallback handling in write-mode overloads where pybind11 could route file-like objects to the py::object filename overload

Test plan

• Verified pathlib.Path works end-to-end for reading and writing WAV files
• Verified AudioFile(Path(...)) works for both read and write modes
• All existing file-like object tests pass (pre-existing test_write_to_seekable_stream failures also present on master)
• test_read_from_end_of_stream_produces_helpful_error_message passes locally

Closes #451

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