refactor(archiver): split L1 checkpoint ingestion into phases

[spalladino]

Splits the archiver's L1 checkpoint ingestion into named phases in a new CheckpointIngestor, without changing behavior apart from one log-context trim.

Context

ArchiverL1Synchronizer.handleCheckpoints had become one 350-line loop doing batch arithmetic, calldata retrieval, promotion lookup, attestation screening, rejected-ancestor tracking, blob fetching, persistence with the sync-point rewinds, and prune events. It shared state through a mutable RollupStatus bag, and the rule that picks which invalid checkpoint to report for invalidation could only be tested by running the whole archiver.

Approach

• PendingChainValidationTracker owns the "first invalid checkpoint" rule and the decision of which status to persist per batch. It has its own unit tests.
• Matching L1's last checkpoint against the local proposed one returns a tagged union (promote / evict / none). The updater's addCheckpoints takes the promoted PublishedCheckpoint directly instead of an object that repeated its fields.
• RollupStatus holds only the rollup's four values, with Fr archives. The validation status, added blocks and last seen checkpoint are returned explicitly, and the write-only lastRetrievedCheckpoint is gone.
• Each batch runs retrieve → plan (match, screen, build) → persist → log. The batch sync-point guard from fix(archiver): restore the L1 sync point when a checkpoint batch fails #336 keeps its extent, read point and persisted point. The first-gap rewind and the batch guard are now separate named methods.
• These phases now live in CheckpointIngestor, the checkpoint counterpart of InboxMessageSynchronizer. The move is its own commit. Reorg reconciliation stays in the synchronizer.

The only behavior change is log context: the "Failed to reach checkpoint … Rolling back" warn no longer carries validationResult, blocksAdded and lastRetrievedCheckpoint, which serialized whole blocks into one line. The rollup-status trace likewise no longer carries validationResult and blocksAdded.

New tests in archiver-sync.test.ts cover the recovery paths directly:

• the gap rewind falling back to the rollup start block;
• eviction and equivocation when L1's checkpoint is also rejected;
• promoted blocks not being reported as added;
• the validation status across batches;
• partial progress when a later batch fails;
• prune-event visibility;
• rollback to the last checkpoint seen in a pass;
• the reconciliation early return and the status after an unwind.

Each commit is reviewable on its own.