ZIP 302: Add support for Structured Memos

[str4d]

This is the TLV format that has been long-desired and not yet finished.

[linear]

COR-1074

[Zk-nd3r]

NSM1 (PR #1243) is a concrete use case for this. We're writing structured lifecycle attestation data to Orchard memos today - event types, BLAKE2b hashes, Merkle root commitments. The deployed system uses a transitional binary layout in the raw 512-byte memo field.

Once ZIP 302 lands, NSM1 attestation payloads would be encoded as a partType. The attestation semantics (event registry, hash construction, Merkle rules, verification procedure) are independent of the container.

Already noted the relationship in the NSM1 draft and in a comment on PR #1243.