fix(archiver): validate checkpoint attestations from calldata before fetching blobs (A-1252)

[spalladino]

Summary

Re-scoped replacement for #24183. During L1 sync the archiver fetched and decoded a checkpoint's blob data before validating its committee attestations. A checkpoint published with invalid/insufficient attestations and malformed-but-hash-matching blob data threw BlobDeserializationError during decode before the invalid-attestation rejection path ran — so it was never recorded as rejected, the L1 sync point never advanced past it, and the archiver re-queried the same L1 blocks every poll and re-threw forever, taking any valid checkpoints in the same batch down with it.

Fixes A-1252

Fix

Validate attestations from L1 calldata first. The signed consensus payload (header, archive root, fee asset price modifier) is fully available from calldata without any blob, so a checkpoint with invalid attestations (or one descending from an already-rejected ancestor) is rejected — emitting the same events and persisting the same rejected entries as before — without fetching its blob. Blobs are then fetched only for the surviving (attestation-valid, non-descendant) checkpoints.

• validation.ts: add validateCheckpointAttestationsFromCalldata and extract the shared core validator (validateCheckpointAttestations now delegates to it). The calldata-built ConsensusPayload is identical to the blob-decoded one, so no accept/reject verdict changes.
• l1_synchronizer.ts: reorder handleCheckpoints to validate-then-fetch; lastSeenCheckpoint is tracked from calldata since rejected checkpoints are no longer built into PublishedCheckpoints.

What changed vs #24183

This PR is narrower than #24183. It drops all of #24183's blob-failure skipping machinery (the "rows 4/5" work: the BlobFetchOutcome sentinel, the canPrune-gated skip, and the stopAfterBatch early loop break).

A blob fetch/decode failure for a checkpoint that has valid attestations stays fatal — it throws and propagates, rolling back the L1 sync point so the fetch is retried on the next iteration, exactly as before #24183. We do not skip such checkpoints.

Deferred to A-1260

Rather than skipping checkpoints if they are prunable, which would lead to the proposer trying to build on top of an incorrect chain tip from the rollup's point of view (since the validators' archive skips checkpoints that the rollup on L1 still considers valid until it's actually pruned), we keep the archiver stuck in them to prevent advancing past an invalid point. To ensure they get pruned, we add a code path in the sequencer so it attempts a prune even when syncing fails, piggybacking on the "vote if propose failed" path.

Tests

• archiver-sync.test.ts: regression test — a checkpoint with invalid attestations and malformed blob data is rejected without a BlobDeserializationError retry loop, and re-polling the same L1 state is stable. Plus a test that a malformed blob with valid attestations still throws (stays fatal), and that a matching local proposed checkpoint is promoted even when its on-chain blob is malformed (blob fetch skipped).
• epochs_invalidate_block.parallel.test.ts: a canonical invalid-attestations checkpoint is rejected from calldata before any blob fetch — a late observer (blob withheld, promotion disabled) syncs past it without ever ingesting it, and once an honest proposer invalidates and replaces it, the on-chain archive at that number differs from the bad checkpoint's and every node progresses past it.

Full archiver-sync.test.ts (56) and validation.test.ts (14) pass; yarn build / format / lint clean.