Run test matrix on macOS/Windows and normalize readonly fixture newlines

[Copilot]

This updates the test workflow to run across Linux, macOS, and Windows, which exposed newline-sensitive assertions in readonly-directory tests. The test fixtures now write LF line endings explicitly so the same assertions hold across platforms.

• Workflow

  • Add an OS matrix to the Test workflow:
    • ubuntu-latest
    • macos-latest
    • windows-latest
  • Keep the existing Python version matrix unchanged.

• Test fixture normalization

  • Update readonly-directory test fixtures to write files with explicit \n newlines.
  • This removes Windows-only \r\n output differences when guest code reads host-provided files.

• Affected tests

  • tests/test_replay_session.py
  • tests/test_runtime_semantics.py

(tmp_path / "hello.txt").write_text("hello from wasi\n", newline="\n")