Bound monotonic timestamp drift; return receipt_summary from verify
monotonic_now: increment changed from 1μs to 1ms, drift capped at 300s.
Rejects registration if clock drift exceeds cap instead of silently
issuing synthetic timestamps indefinitely.
verify/verify_receipt_only now return receipt_summary with total,
verified, failed, skipped counts so callers can detect partial
verification (e.g. 2 of 3 receipts tampered).