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).
- Add READ_TIMEOUT (5 min) and MAX_OUTPUT_SIZE (100 MB) constants
- Implement read_pipe_with_timeout() with watchdog thread protection
- Update execute_grep_command() to capture and return both streams
- Prevents TH-03 hang scenario by bounding both time and memory
Acceptance criteria:
✓ Stdout captured completely
✓ Stderr captured completely
✓ Output buffered without truncation (100 MB limit)
✓ Read operations have timeout protection (5 min timeout)
✓ Both streams captured even if one is empty
Verification: notes/bf-3wkpz.md
