feat(consensus): admit four-mebibyte protected session values end-to-end

[VerifiedOrganic]

Problem

The durable SQLite session backend can support protected records with a 4 MiB plaintext ceiling plus 64 KiB of envelope/framing reserve, but the consensus profile cannot truthfully advertise that capacity end to end:

• CONSENSUS_MAX_RPC_PAYLOAD_BYTES is 2 MiB, so a larger CAS command is rejected before Raft proposal.
• Consumer batch response admission budgets four JSON bytes per stored payload byte under an 8 MiB aggregate ceiling, so raising the consensus value capability would reject every Get/CAS batch slot.
• Network frames remain independently bounded and must stay consistent with both request and response ceilings.

This is a generic session-consensus transport/capability gap. Product-specific code must not bypass it or invent a local store/transport substitute.

Required scope

• Define one explicit maximum protected session value supported by consensus, including command, replication, snapshot, restore, and consumer response framing.
• Make ConsensusSessionStore::capabilities().max_value_bytes truthful for that complete path.
• Admit a protected 4 MiB plaintext record plus bounded envelope/framing overhead through a real multi-node proposal and exact readback.
• Preserve bounded allocation, batching, cancellation, redaction, and fail-closed oversize behavior.
• Keep point operations usable without making small Get/CAS batches unconditionally fail merely because the backend supports a larger value.
• Reject exact one-over request/response/replication boundaries before mutation.

Evidence

• Unit boundary tests for consensus codec/RPC and consumer response admission.
• Exact-at-limit and one-over proposal tests.
• Multi-node encrypted CAS/readback/restore proof.
• Mutation proving that lowering any required transport or response fence makes the acceptance test RED.

Refs #663.

[VerifiedOrganic]

PR #684 improved protected storage limits but explicitly excluded the end-to-end 4 MiB consensus command/RPC scope. This issue remains open.