Scan time budget reads the CPU clock twice per pattern check #93

Every pattern check pays for the scan time budget: scanPatternCharged calls getrusage() twice per pattern check (allocating an array each time) whenever a budget is active, and the performance monitor sanitizes each anomaly before it knows whether any callback is registered. The Python reference engine fixed the same overhead in Guard-Core/guard-core#172. I am charging the budget per pass at 2 ms slice boundaries and skipping the sanitized copy when there are no callbacks.