Delete dead cbor helpers; Error on Ptime.max; best-of-N receipts; root in export

- Remove unused cbor_bytes/cbor_int/cbor_text (superseded by local
helpers in check_receipt_leaf).
- monotonic_now returns Error instead of duplicate timestamp when
Ptime.add_span overflows at Ptime.max.
- verify_receipts: skip failed trusted receipts (log warning) instead
of short-circuiting. Succeed if any trusted receipt verifies.
Matches the cross-signing documentation.
- Export includes root hash; import verifies it after replay. Detects
entry reordering or truncation.