Design DA for l-BFV key material

[0xjei]

Parent: #1736
Related work: #1959

Context

If l-BFV key material uses the classic TrBFV on-chain publication path, the current estimate is about 80 shares or chunks to post for one E3. Confirm what this count represents and measure the resulting calldata, gas, and publication latency before choosing a design.

Issue #1959 tracks data-availability options for latency-sensitive applications. Determine whether that work covers the l-BFV key flow or whether it needs a separate design.

Scope

• Measure the serialized size and number of records or transactions for each l-BFV artifact under the current path.
• Evaluate the current on-chain path and suitable data-availability designs with issue Explore DA solutions for latency-sensitive applications #1959.
• Define how consumers retrieve the data and how the protocol checks that it matches the existing proof and commitment.
• Define availability, retry, and recovery requirements for the l-BFV flow.
• Record the design decision and create implementation and validation follow-up issues.

This issue tracks the design decision. Do not select an implementation until the data size, cost, latency, and availability requirements are clear.

Acceptance criteria

• The estimate of about 80 posts is confirmed or corrected, with its units and assumptions recorded.
• The current path and candidate data-availability designs have measured cost and latency estimates.
• The selected design preserves the existing proof and commitment checks and states how consumers retrieve and validate the data.
• Implementation and end-to-end validation work is tracked in follow-up issues.