fix(archiver): resolve L2-to-L1 witness from a single store snapshot

[spalladino]

getL2ToL1MembershipWitness assembled its witness from several non-atomic archiver reads (getTxEffect, then the epoch blocks, target block and checkpoint metadata read inside computeL2ToL1MembershipWitness). A store commit landing between those reads could splice together two different chain states and surface a spurious "message does not exist" error even when the tx-effect index was healthy.

Wrap the witness assembly in a single store.transactionAsync so every archiver read binds to one write transaction and observes a consistent snapshot (the store serializes writers, so no commit can interleave). The L1 Outbox roots fetch stays outside the transaction to avoid holding the archiver's writer lock across a network round-trip, and the tx effect is re-read inside the snapshot so the receipt's block number and tx index stay consistent with the block data.

Cost: witness assembly briefly serializes with archiver writers (it holds the store's writer queue while it reads), but the work under the lock is pure store reads plus an in-memory tree build — no network I/O.

Context: this came out of the A-1426 investigation. The incident symptom itself turned out to be a downstream client bug following a v5 behavior change in L2BlockSynchronizer — not this race, and not archiver index corruption. This PR is therefore hardening against a real but so-far-unobserved torn-read window, not the incident fix. See #24765 for the companion store hardening from the same investigation.

Related to A-1426