fix(cannon): fall back to retained states for committee fetches on pruned beacons

Pruned beacons drop epoch-boundary states at the finalized split, which
permanently starved the elaborated-attestation and sync-aggregate derivers.
Fetch epoch N committees from the epoch N+1 boundary state (the retained
split), and the sync committee from the finalized state with the epoch
pinned so a period mismatch errors instead of returning wrong data.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: Savid

pkg/cannon/deriver/beacon/eth/v2/beacon_block_sync_aggregate.go

@@ -457,7 +457,17 @@ func (b *BeaconBlockSyncAggregateDeriver) fetchSyncCommittee(
 		State: fmt.Sprintf("%d", boundarySlot),
 	})
 	if err != nil {
-		return nil, errors.Wrap(err, "failed to get sync committee from beacon node")
+		// Pruned (non-archive) beacons drop the period-boundary state, but every
+		// retained state in the period carries the same sync committee. Pinning
+		// the epoch makes the node error on a period mismatch rather than
+		// silently returning a different period's committee.
+		resp, err = provider.SyncCommittee(ctx, &api.SyncCommitteeOpts{
+			State: "finalized",
+			Epoch: &epoch,
+		})
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to get sync committee from beacon node")
+		}
 	}
 
 	if resp == nil || resp.Data == nil {

pkg/cannon/ethereum/services/duties.go

@@ -137,9 +137,15 @@ func (m *DutiesService) FetchBeaconCommittee(ctx context.Context, epoch phase0.E
 
 	committees, err := m.beacon.FetchBeaconCommittees(ctx, fmt.Sprintf("%d", phase0.Slot(epoch)*spec.SlotsPerEpoch), &epoch)
 	if err != nil {
-		m.log.WithError(err).WithContext(ctx).Error("Failed to fetch beacon committees")
-
-		return nil, err
+		// Pruned (non-archive) beacons drop this epoch's boundary state once the
+		// epoch finalizes, but the next epoch's boundary state — the finalized
+		// split, which pruning retains — still carries this epoch's shuffling.
+		committees, err = m.beacon.FetchBeaconCommittees(ctx, fmt.Sprintf("%d", phase0.Slot(epoch+1)*spec.SlotsPerEpoch), &epoch)
+		if err != nil {
+			m.log.WithError(err).WithContext(ctx).Error("Failed to fetch beacon committees")
+
+			return nil, err
+		}
 	}
 
 	m.beaconCommittees.Set(epoch, committees, time.Minute*90)