observer: blank-slate branch — keep wiring, remove detectors/correlators

Starting point for novel detector/correlator ideas. Keeps the full observer
pipeline wiring (engine, scheduler, storage, fetcher, catalog framework,
testbench, HTMLReporter, extractors, interfaces in comp/observer/def) and
removes every concrete detector and correlator implementation.

Removed:
  - Detectors: CUSUM, BOCPD, RRCF (+ rrcf tree lib), ScanMW, ScanWelch
  - Correlators: cross_signal, time_cluster, passthrough
  - engine_stepadvance_test (relied on TimeClusterCorrelator)
  - TimeCluster-specific handlers/const in HTMLReporter
  - Unused statistical helpers in metrics_detector_util.go (kept seriesStatus
    + bulkSeriesStatus, which storage.go depends on)

Catalog still contains the extractors (log_metrics, connection_error,
log_pattern); the detector/correlator slots are empty. Add new entries
alongside new impls.

comp/observer/... builds and tests cleanly.

Author: ellataira