feat: event-loop lag monitor with sampling profile (closes #714)

[laurentketterle-hub]

Description

Implements event-loop lag monitor with sampling profile for capacity/saturation alerts.

Closes #714

Changes

• New module: src/monitoring/event_loop_lag.py — Event-loop lag monitor with:
  • Configurable SamplingProfile (window size, sample interval, thresholds)
  • Real-time degradation detection with consecutive-sample tracking
  • Three alert levels: warning (100ms), critical (500ms), saturation (1000ms)
  • P50/P95/P99 percentile statistics
  • Thread-safe sampling with threading.Lock
  • Start/stop/reset lifecycle
  • Customizable alert callbacks (on_warning, on_critical, on_saturation)
• Tests: tests/test_event_loop_lag.py — 13 tests covering:
  • Initial state, start/record/stop lifecycle
  • Lag detection and measurement accuracy
  • Stats aggregation (min, max, avg, percentiles)
  • Degraded/critical/saturation callbacks
  • Thread safety (via lock)
  • Reset behavior

How to verify

python -m pytest tests/test_event_loop_lag.py -v