Integration tests across multiple FFmpeg versions

## Goal

Test the library against multiple FFmpeg versions to ensure compatibility and catch version-specific regressions.

## Current State

- CI runs on **JDK 11, 17, 21** but only one FFmpeg version (`release` via `setup-ffmpeg`).
- Unit tests mock FFmpeg — they don't exercise real FFmpeg behavior.
- No integration tests that verify actual transcoding, probing, or option handling against a real FFmpeg binary.

## Plan

### 1. Add FFmpeg version matrix to GitHub Actions
- Extend the CI matrix to include multiple FFmpeg versions (e.g. `5.1`, `6.1`, `7.1`, `release`).
- Use `FedericoCarboni/setup-ffmpeg` which supports version pinning, or download specific builds from ffmpeg.org.
- Matrix: `{java: latest} x {ffmpeg: [5.1, 6.1, 7.1, release]}`.

### 2. Create integration test suite
- New test class(es) under `src/test/java/.../integration/`, gated behind a Maven profile (e.g. `-Pintegration`) so they don't run in normal unit test builds.
- Tests should cover:
- **Probing**: `FFprobe.probe()` against sample files — verify parsed codecs, streams, format, duration.
- **Transcoding**: Basic transcode (e.g. mp4 to mkv, audio extraction) — verify output file is valid.
- **Builder options**: Exercise every builder option (`setVideoCodec`, `setAudioCodec`, `setFormat`, `setVideoFilter`, `setComplexFilter`, `setFrames`, HLS output, etc.) and verify the resulting FFmpeg command succeeds.
- **Progress listener**: Run a real transcode with `ProgressListener` and verify progress events are received.
- **Error handling**: Feed invalid input and verify exceptions are thrown with useful messages.
- **Edge cases**: Unicode filenames, paths with spaces, large filter strings.

### 3. Sample media files
- Include small (< 1MB) sample media files in `src/test/resources/` for integration tests.
- Cover common formats: mp4 (h264+aac), mkv, webm, mp3, wav, etc.

### 4. CI workflow update
- Add a separate job or workflow (`integration-test.yml`) that runs the integration profile.
- Only run on `push` to main and PRs (not on every commit to avoid long CI times).

This is related to #31 (documenting supported FFmpeg versions) — once we have the test matrix, we can confidently document which versions are supported.