Add comprehensive metric extraction logic for benchmark output parsing:
- Add RawTimingMetrics struct with fields for wall time, CPU time, file counts, and throughput
- Implement extract_raw_timing_metrics() to parse stdout/stderr for timing information
- Add extract_time_value() helper to parse CPU time from text formats
- Update run_benchmark() to use extraction logic instead of simple parsing
- Calculate throughput metrics (files/sec, MB/s) from extracted data
Closes bf-5b8mk. Verification: notes/bf-5b8mk.md, commit $(git rev-parse --short HEAD).
