feat: block proposals carry a bucket reference (A-1381)

[spalladino]

Adds an optional InboxBucketRef (bucketSeq, bucketTimestamp, inboxRollingHash) to BlockProposal and to the last block of CheckpointProposal (AZIP-22 Fast Inbox), so validators can derive the consumed L1-to-L2 message bundle from their own Inbox view instead of trusting a proposer-supplied list. Plumbing only for now — the sequencer passes undefined pre-flip (populated in FI-12) and the legacy validation path ignores it (validated in FI-13).

• Signed, not an unsigned lookup aid: the reference lives inside the signed payload (getPayloadToSign), so a relay cannot strip or inject it without breaking signature recovery. The checkpoint header's inboxRollingHash remains the consensus commitment; a wrong reference can only miss the Inbox lookup. This discharges the P2-2 residual from the A-1371 design (signedness + republish path).
• Optional-tail wire encoding, no topic-version bump: an unset proposal serializes byte-identically to the legacy format, and decoders treat EOF after the existing fields as "no reference", so mixed-version gossip keeps working. Gossip topic versions derive from deployment identity, not node software version, so mixed decoders are a hard requirement.
• Wire-compat evidence: golden fixtures captured from the pre-change bytes pin (a) unset serialization is byte-identical, (b) legacy buffers decode as no-reference, for both proposal types.
• CheckpointProposal enforces at construction that a set last-block reference's rolling hash equals the checkpoint header's inboxRollingHash, mirroring the existing inHash cross-check.
• Audited every BlockProposal.fromBuffer / CheckpointProposal.fromBuffer decode site in p2p/src and message_validator.ts: none assume a strict buffer length, so the appended tail is tolerated.

Tests: InboxBucketRef round-trip/equals/schema; block + checkpoint proposal round-trips with the reference set (with and without signed txs); byte-identity vs golden fixtures when unset; legacy-buffer tolerance; signature coverage (recovery over a set reference, and recovery breaks when the reference is tampered with or injected); payload hashes differ set-vs-unset; checkpoint header-consistency check throws on mismatch.