fix(archiver): avoid loading full blocks during checkpoint ingestion

[spalladino]

Stack

Part of A-1817. Bottom to top, each PR targets the branch below it:

• fix(archiver): avoid loading full blocks during checkpoint ingestion #25279 spl/faster-block-ingestion -> merge-train/spartan-v5 — cheaper checkpoint ingestion, so writes hold the store for less time <- this PR
• feat(kv-store): expose read-only lmdb transactions #25280 spl/kv-store-read-only-tx -> spl/faster-block-ingestion — adds readOnlyTransaction plus txId on the native GET / START_CURSOR
• fix(archiver): batch multi-key point reads into one LMDB round trip #25282 spl/batched-kv-gets -> spl/kv-store-read-only-tx — adds getMany / getManyAsync; depends on feat(kv-store): expose read-only lmdb transactions #25280 for txId on GetRequest
• fix(archiver): speed up log-by-tag queries #25254 spl/faster-get-private-logs-by-tags -> spl/batched-kv-gets — faster tag scans; depends on fix(archiver): batch multi-key point reads into one LMDB round trip #25282 for getManyAsync on the log store's per-block reads
• fix(archiver): serve log-by-tag queries from read-only snapshots #25315 spl/log-store-read-only-snapshots -> spl/faster-get-private-logs-by-tags — moves the log store's reads onto snapshots; depends on feat(kv-store): expose read-only lmdb transactions #25280 for readOnlyTransaction

Context

The archiver ingests each checkpoint inside a single transactionAsync, so ingestion duration directly delays any read queued behind it (notably getLogsByTags, see #25254). Profiling showed the single biggest cost was not the write itself: getPreviousCheckpointBlock fetched the previous checkpoint's last block as a full L2Block — one sequential native LMDB round trip plus deserialization per tx effect — when its only consumer, validateCheckpointBlocks, reads four header-level fields.

Approach

• getPreviousCheckpointBlock now reads the lightweight block data (as addProposedBlock already does) and returns just the chaining info (number, checkpointNumber, indexWithinCheckpoint, archive). L2Block satisfies the shape structurally, so the validation body, error types, and messages are unchanged.
• LogStore.addLogs serialized the same txHash once per log and the same blockHash once per log in the whole block; both are now serialized once per tx / once per block and passed to encodeValue pre-serialized. The stored byte format is unchanged.

Measured A/B (per-checkpoint ingestion addCheckpoints + addLogs, LMDB on NVMe, mean of 10 checkpoints after warmup, two runs each):

txs/block	before	after	delta
36	77.7 / 86.5 ms	42.3 / 41.1 ms	≈ −49%
120	248.1 / 244.0 ms	140.8 / 142.4 ms	≈ −42%

[spalladino]

Closing: this stack rotted after the repo split. The native read-only transaction support is being redone in aztec-packages (native-packages/kvdb), and the archiver/kv-store TS changes are being redone in aztec-node.