Detection costs more CPU per request in 4.4.0 than in 4.3.x #172

Since 4.4.0 every request costs noticeably more CPU, even when requests run one at a time. On a realistic POST (JSON body, browser headers, a bearer token) I measured about 209 ms of CPU per request on 4.4.0 against about 148 ms on 4.3.1, with the same number of pattern checks. The per-request scan time budget reads the thread CPU clock twice and enters a context manager for every pattern check (about 57k clock reads per request), and the 2 ms event-loop slice restarts for every scanned value, so it rarely fires and responsiveness comes from per-pattern thread pool round trips instead. Separately, the performance monitor recomputes the variance with a per-element Python loop on every pattern check and sanitizes anomalies even when no callback is registered. I am charging the budget once per pass with a request-scoped slice and making the monitor statistics cheaper, without changing any detection result.