fix(checkpoints): discover checkpoints from latest OL epoch

[krsnapaudel]

Discover checkpoints from the latest OL epoch

Problem

The explorer gated checkpoint discovery on strata_getChainStatus().confirmed.epoch, fetching only start..=confirmed. A checkpoint sealed on the OL but not yet L1-confirmed (latest > confirmed) was never fetched, so it stayed invisible in the explorer until confirmed caught up — the observed checkpoint lag (fullnode_latest=2 while checkpoint 3 was already sealed). Ref: STR-3652

Fix

• Discover checkpoints up to latest.epoch instead of confirmed.epoch; keep confirmation_status from getCheckpointInfo as the source of truth for display status.
• Deserialize latest, confirmed, and finalized from getChainStatus using strata_identifiers::EpochCommitment, pinned to the same strata-common tag used by ../alpen.
• Require the full chain-status shape; no fallback to confirmed if latest is absent.
• Point the block fetcher at the latest checkpoint we attempted to sync, falling back to the highest checkpoint actually in the DB when the latest checkpoint is not stored yet.
• Log fullnode_latest, fullnode_confirmed, and fullnode_finalized.

Testing

• mock_fullnode now returns full epoch commitments for all three slots (latest = n−1 reported as Pending), exercising the pending-checkpoint discovery path.

Note / follow-up

Deserialization is now strict: each epoch slot requires epoch + last_slot + last_blkid. Confirm the live strata_getChainStatus returns full EpochCommitment objects before relying on this in production.

[storopoli]

ACK 85954bb

1 non-blocking comment.