`test_demux_flac_uses_tools_dir_fallback` hard-coded `tools_dir / "ffmpeg"`
in its `fake_exists` stub, but `_demux_flac` looks for `ffmpeg.exe` on
Windows (os.name == 'nt'). Result: the fake_exists stub never matched,
the code fell through to the "ffmpeg is required" RuntimeError instead
of the expected "ffmpeg failed" subprocess error, and the test failed
on Windows. Linux CI passed because os.name == 'posix' uses bare
"ffmpeg".
Pick the binary name based on `os.name` to match what `_demux_flac`
actually probes for. Asserts on the matching candidate path.
Tests: 20 passing on Windows (was 19/20). Ruff clean.
