fix(fast-inbox): synthesize the genesis Inbox bucket on read (A-1383)

The archiver only writes bucket snapshots for ingested messages, whose first bucket is
sequence 1, so `getInboxBucket(0)` / `getInboxBucketByTotalMsgCount(0)` returned undefined.
That broke streaming resolution of a genesis parent (first block would reject
`parent_bucket_unresolved`), of an empty genesis block's own bucket reference, and let an
empty checkpoint skip the last-block censorship enforcement. Synthesize the sequence-0
sentinel (rolling hash 0, total 0) on read, mirroring the on-chain Inbox's base case, with
a store-level test exercising it through a populated MessageStore.

Author: spalladino

yarn-project/archiver/src/store/message_store.test.ts

@@ -403,6 +403,14 @@ describe('MessageStore', () => {
       expect(await messageStore.getInboxBucketByTotalMsgCount(7n)).toBeUndefined();
     });
 
+    it('synthesizes the genesis sentinel bucket (sequence 0, total 0) which is never ingested', async () => {
+      // With real messages present but no ingested sequence-0 snapshot, both lookups still resolve genesis.
+      await messageStore.addL1ToL2Messages(makeBucketedMessages(threeBucketSpec));
+
+      expect(await messageStore.getInboxBucket(0n)).toMatchObject({ seq: 0n, totalMsgCount: 0n, msgCount: 0 });
+      expect(await messageStore.getInboxBucketByTotalMsgCount(0n)).toMatchObject({ seq: 0n, totalMsgCount: 0n });
+    });
+
     it('continues a bucket that spans two insertion batches', async () => {
       const msgs = makeBucketedMessages(threeBucketSpec);
       await messageStore.addL1ToL2Messages(msgs.slice(0, 2));

yarn-project/archiver/src/store/message_store.ts

@@ -54,6 +54,20 @@ function deserializeBucketSnapshot(buffer: Buffer): BucketSnapshot {
   return { inboxRollingHash, totalMsgCount, timestamp, msgCount, lastMessageIndex };
 }
 
+// The genesis sentinel bucket (AZIP-22 Fast Inbox): sequence 0 with a zero rolling hash and no messages, mirroring the
+// on-chain Inbox's base case. The archiver never ingests a snapshot for it (no message is absorbed into sequence 0), so
+// it is synthesized on read. Consumers use its sequence number and zero total; its deploy-time timestamp is not tracked
+// here and is unused.
+const GENESIS_INBOX_BUCKET: InboxBucket = {
+  seq: 0n,
+  inboxRollingHash: Fr.ZERO,
+  totalMsgCount: 0n,
+  timestamp: 0n,
+  msgCount: 0,
+  lastMessageIndex: 0n,
+  isOpen: false,
+};
+
 export class MessageStoreError extends Error {
   constructor(
     message: string,
@@ -403,11 +417,16 @@ export class MessageStore {
 
   /**
    * Returns the Inbox bucket with the given sequence number, or undefined if it has not been synced (AZIP-22 Fast
-   * Inbox).
+   * Inbox). Sequence 0 is the genesis sentinel: the on-chain Inbox reserves it as the "consumed nothing" base case
+   * and never absorbs a message into it, so the archiver ingests no snapshot for it; it is synthesized here (rolling
+   * hash 0, total 0) so streaming consumers can resolve a genesis parent or an empty checkpoint's last-consumed bucket.
    */
   public async getInboxBucket(seq: bigint): Promise<InboxBucket | undefined> {
     const snapshot = await this.getBucketSnapshotBySeq(seq);
-    return snapshot && this.toInboxBucket(seq, snapshot);
+    if (snapshot !== undefined) {
+      return this.toInboxBucket(seq, snapshot);
+    }
+    return seq === 0n ? GENESIS_INBOX_BUCKET : undefined;
   }
 
   /**