Drift prediction: schedule canary runs when a site's asset fingerprints churn

[vnmoorthy]

Today the Sentinel detects drift after an extraction produces bad evidence. We can usually see a redesign coming: bundle hashes, stylesheet URLs, and class-name vocabularies churn hours before content pages flip.

Proposal

• On each run, fingerprint cheap structural signals (script/style URL set, class-vocabulary shingle hash, DOM-depth histogram of item cards).
• When churn exceeds a threshold, schedule an immediate out-of-band canary run instead of waiting for the next autopilot tick.
• Surface a predicted-drift event level in the stream and webhooks.

Constraints (see ADR-0001): prediction may only schedule work earlier — it must never lower the Sentinel's evidence bar or auto-deploy anything.