Anchor a proof reference to every attestation #235

Priority

High — must be addressed in current sprint

User story / Problem statement

Currently an attestation carries only the share price and its expiry. Nothing on-chain links that number to the evidence behind it, so a reader has to trust the reporter. The submission promises "Multisig reporter, deviation cap, cooldown, freshness, proof reference" (Tranche 2, deliverable 1), and the design document already states that every attestation carries one. The Tranche 3 SEP-1 template also links the latest proof-of-backing reference, so it depends on this.

Expected outcome

Every accepted attestation stores a proof reference: a hash of the supporting document and a URI where it can be fetched. Integrators read it next to the price.

Acceptance criteria

• NavReport carries a proof hash (BytesN<32>) and a bounded-length URI
• attest rejects an empty hash
• latest() returns the proof reference, and the attestation event emits it
• the admin panel's attest form collects both fields
• tests cover acceptance, rejection and the event payload
• the design document and the code describe the same report shape

Alternatives considered

#173 point 3 proposes dropping the proof reference from the design document, since nothing verifies it on-chain, and favouring a second confirmation for anomalous reports. Dropping it means amending a funded Tranche 2 deliverable and the Tranche 3 SEP-1 template with SCF. The two are not exclusive: the reference is cheap; the confirmation can follow.

Technical notes

The contract anchors the proof; it never validates it. Changing NavReport changes the generated bindings consumed by the attest form merged in #224.