tests: test_env_override_wins hardcodes Linux ffmpeg path on macOS

## Summary While validating PR #107 on macOS, I found that `test_env_override_wins` hardcodes `/usr/bin/ffmpeg`. The implementation correctly rejects nonexistent override paths, so the test fails on supported macOS installations where FFmpeg is normally elsewhere (for example, Homebrew uses `/opt/homebrew/bin/ffmpeg` on Apple Silicon). ## Environment - HFlow upstream `main` (verified against the current raw source on 2026-08-22) - macOS, Apple Silicon - Python 3.14.2 - uv 0.12.5 - FFmpeg installed and available on `PATH`, but not at `/usr/bin/ffmpeg` ## Steps to reproduce ```bash uv sync --locked --all-extras uv run pytest -q tests/test_ffmpeg.py::test_env_override_wins ``` ## Expected behavior The platform-supported test should verify that an existing override path wins without depending on a Linux-specific filesystem location. ## Actual behavior ```text E hflow.ffmpeg._binary.FfmpegNotFoundError: E HFLOW_FFMPEG=/usr/bin/ffmpeg does not exist 1 failed ``` The failure is deterministic because `ffmpeg_path()` intentionally checks `Path(override).is_file()`. ## Possible cause `tests/test_ffmpeg.py:67` sets: ```python monkeypatch.setenv(FFMPEG_ENV_VAR, "/usr/bin/ffmpeg") ``` That path is not portable to macOS, despite CONTRIBUTING.md listing macOS as a supported native development platform. ## Proposed solution Create a temporary file with `tmp_path`, set `HFLOW_FFMPEG` to that path, and assert the resolver returns it. The test only exercises path precedence, so it does not need a real executable: ```python def test_env_override_wins( tmp_path: Path, monkeypatch: pytest.MonkeyPatch, cleared_binary_caches: None, ) -> None: override = tmp_path / "ffmpeg" override.touch() monkeypatch.setenv(FFMPEG_ENV_VAR, str(override)) assert ffmpeg_path() == override ``` This keeps the test hermetic and platform-independent.