GuardLog: JSON-serializable audit record with text hashes (never raw
text), scanner chain results, finding types. Emitted via onScan callback.

GuardMetrics: per-run timing snapshot (total/input/output duration),
block status, per-scanner finding counts. Emitted via onMetrics callback.

AiGuard.fromConfig: build scanner chains from Map<String, dynamic>.
Supports all 13 scanner types. JSON via dart:convert, zero new deps.