Preserve source video FPS in CLI track output

[SkalskiP]

Summary

• Output videos were always written at 30 FPS regardless of the source frame rate. Now _VideoOutput receives the actual source FPS from _classify_source and falls back to _DEFAULT_OUTPUT_FPS (30.0) for sources without a known frame rate (e.g. image directories).
• Extracted _DEFAULT_OUTPUT_FPS constant to eliminate the magic number.
• Cleaned up video.py: renamed h, w to height, width, added return type annotations to __enter__/__exit__, removed forbidden module-level docstring.