EP-09 Add CLI argument parsing: --encoder, --preset, --bitrate, --crf, --input, --output #12

Replace hardcoded params with CLI args. Support --encoder, --preset, --crf, --bitrate, --input, --output, --help.

Why

Tool must be usable without recompiling. CLI args enable scripted benchmark runs.

Acceptance criteria

• --encoder x264|x265
• --preset 1-9
• --crf 0-51 (mutually exclusive with --bitrate)
• --bitrate kbps
• --input filepath or 'testsrc'
• --output JSON path
• --help prints usage with defaults
• Invalid args exit with code 1