feat: add detector benchmark script

[sanmaxdev]

Summary

• Add a detector latency and throughput benchmark script for reusable local measurements.
• Support built-in sample data, JSONL benchmark cases, warmup runs, repeated measurements, and JSON output.
• Cover benchmark timing and input validation in the existing inference test suite.

Testing

• python3 -m pytest tests/test_inference_pytest.py -v -k "not TestAnswerStartToken"
• python3 -m ruff format --check --diff lettucedetect/ tests/
• python3 -m ruff check lettucedetect/ tests/ --extend-exclude lettucedetect/integrations/
• python3 -m ruff check scripts/benchmark_detectors.py tests/test_inference_pytest.py
• python3 scripts/benchmark_detectors.py --help

Closes #58