Improve GitHub Actions workflow summary output with dedicated baseline
comparison table and human-readable metric formatting.

Changes:
- Add baseline_meta field to Report for storing baseline run metadata
- Generate comparison header showing current vs baseline commits
- Add baseline comparison table with performance deltas
- Format verification expressions with proper unit scaling
- Support numeric literals in expressions (integers, decimals, scientific)
- Add comprehensive test coverage for formatting functions

The verification table now shows resolved metric values (e.g., "rps = 3.38M req/s")
instead of raw numbers, making reports more readable for humans.