Add performance benchmarks to CI #19

Run bench.php in CI to catch overhead regressions. Report:

• Vanilla PHP (no extension)
• Extension loaded, no trigger
• Extension loaded, trigger present, no client

What to do

1. Add a benchmark job to a separate benchmark.yml
2. Run bench.php 5× in each configuration, report median
3. Fail CI if "loaded, no trigger" overhead exceeds 6% (currently ~4%)
4. Store results as CI artifact for historical tracking

Acceptance criteria

• Benchmark runs in CI on every PR
• Results visible in CI output
• Threshold check: fails if overhead > 6%