Add scene-aware engine bakeoff planning

[shiny-code-bot]

Summary

• add mediaforce bakeoff to write a repeatable scene-aware engine bakeoff JSON plan from a run manifest
• carry size-first defaults and resolved per-item policies into the bakeoff plan
• list current ab-av1 plus Av1an, Xav, and Auto-Boost candidates with required tools, command status, source links, and result fields to collect
• document the bakeoff workflow in README

Refs #103

Validation

• uv run --with pytest pytest tests/test_bakeoff.py tests/test_ffmpeg.py
• uv run mediaforce bakeoff --help
• bash scripts/pre-commit-check.sh