test(fast-inbox): streaming inbox e2e latency and mid-checkpoint inclusion (A-1385)

[spalladino]

Test-only coverage for the Fast Inbox (AZIP-22) streaming L1→L2 message path (issue A-1385, FI-15) — the behaviours the legacy suite could not express, because pre-flip every message entered at the first block of the next checkpoint.

Stacked on the node/flip phase: #24784 (A-1379) → #24785 (A-1380) → #24786 (A-1381) → #24787 (A-1382) → #24788 (A-1383) → #24789 (A-1384). Base is spl/a-1384-flip-streaming-inbox.

e2e (end-to-end)

New single-node/cross-chain/streaming_inbox.test.ts, on the proven CrossChainMessagingTest fixture (production pipelining sequencer) with a widened slot (36s, 6s blocks → up to ~4 blocks per checkpoint) and minTxsPerBlock: 0:

• Mid-checkpoint inclusion — times a send so the message ages past INBOX_LAG_SECONDS partway through a checkpoint's build, then asserts the inserting block has indexWithinCheckpoint > 0 and that the immediately preceding block did not yet carry the message. Retries with fresh messages if a message happens to age exactly at a checkpoint boundary.
• Latency bound — asserts, slot-denominated, that includingBlockTimestamp − messageL1Timestamp ≤ INBOX_LAG_SECONDS + 2·slotDuration (derived from the deployed constants, not hardcoded), and that it is positive. Wall-clock latency is logged only (A-1178 style), never asserted.
• Message-only block — on a drained pool, asserts the block that consumes the message carries zero tx effects (the FI-05 zero-tx / non-empty-bundle shape) and that the chain keeps proving past it.
• Send-then-consume on the streaming path — consumes a streaming-inserted message with a public tx by its compact leaf index and asserts a second consume reverts (double-spend protection); the insert/consume block relationship is logged.

prover-client

New checkpoint-sub-tree-orchestrator test for a checkpoint whose L1→L2 messages span multiple blocks (a non-first block carries a bundle). Asserts per-block start/end L1→L2 tree-snapshot continuity and slice partitioning (no gap/overlap; block slice = [prevBlockLeafCount, blockLeafCount)) and the isFirstBlock flag. Adds TestContext.makeCheckpointWithMessagesPerBlock to distribute a bundle across a checkpoint's blocks (the single-block-per-checkpoint makeCheckpoint puts every message in the first block).

Validation

• The e2e suite and the prover-client orchestrator test could not be run locally — the local build is broken in noir-protocol-circuits-types/simulator/prover-client from stale circuit artifacts, and end-to-end sits downstream. CI validates both. Every helper, fixture, and API call is grounded on an existing green suite (l1_to_l2.test.ts, cross_chain_public_message.test.ts) and verified against the current interfaces. No packages that build locally (stdlib, blob-lib, ethereum, foundation) are touched.
• Reviewed by codex (gpt-5.6-terra): it caught an incorrect message-sponge-continuity assumption in the prover test (non-first block roots inherit the checkpoint-wide sponge, not the prior block's end sponge); those sponge assertions were removed, leaving the correct L1→L2 tree-snapshot partitioning checks.

Review follow-up (phase-2 final review)

The multi-block-slice orchestrator test now also asserts per-block message-sponge continuity (start empty, each block absorbs exactly its slice, last end equals the InboxParity sponge) and turns its last block into a zero-tx message-only block, exercising the msgs-only block-root wiring fixed in #24789. A stray async in the e2e suite was dropped (lint).