…d comes out

is_binary() and get_starting_chunk() accept str, bytes, and Path, matching
what Python's open() already handles. is_binary_string() takes bytes,
_compute_features() takes bytes and returns list[float], and the decision
tree's is_binary() takes list[float].

Resolves #626 (type hints for public API). Supersedes #627 (stub files,
no longer needed since Python 2 was dropped). Addresses #628 (pathlib.Path
support, which already worked but is now documented in the types).

Key design decisions:
- Inline annotations instead of .pyi stubs, since the project is Python 3.12+
- str | bytes | Path rather than os.PathLike[str] to match how people
  actually call the function (pathlib.Path, not custom PathLike objects)
- No int (file descriptors): technically works via open() but no demonstrated
  use case and no error handling for bad fds