catalog: log ProjectionStore delta applies + greppable snapshot line

Restore the per-batch visibility lost in the cart migration: applyDelta now logs
'delta applied epoch=N (+U/-D, overlay=X)', and the snapshot swap line includes
'snapshot' so it's findable. Deltas were applying silently, which read as 'nothing
happened' on a bulk catalog update (only the debounced snapshot swap was logged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

catalog/consumer.go

@@ -196,7 +196,7 @@ func (s *ProjectionStore[T]) endSnapshot(epoch int64, meta map[string]string) er
 	}
 	s.epoch = epoch
 	s.overlay = make(map[string]overlayEntry[T]) // base now includes everything ≤ epoch
-	log.Printf("catalog: ProjectionStore base swapped epoch=%d (%d entries)", epoch, s.buildingSeen)
+	log.Printf("catalog: ProjectionStore snapshot applied — base swapped epoch=%d (%d entries)", epoch, s.buildingSeen)
 	return nil
 }
 
@@ -222,16 +222,23 @@ func (s *ProjectionStore[T]) applyDelta(epoch int64, payload []byte) error {
 		// this epoch; the consumer's fallback covers reads until then.
 		return nil
 	}
+	var upserts, deletes int
 	for _, u := range ups {
 		if u.SKU == "" {
 			continue
 		}
 		if u.Deleted {
 			s.overlay[u.SKU] = overlayEntry[T]{deleted: true}
+			deletes++
 			continue
 		}
 		// Transform applied here too — overlay holds the consumer's shape.
 		s.overlay[u.SKU] = overlayEntry[T]{val: s.transform(u.Projection)}
+		upserts++
+	}
+	if upserts+deletes > 0 {
+		log.Printf("catalog: ProjectionStore delta applied epoch=%d (+%d/-%d, overlay=%d)",
+			epoch, upserts, deletes, len(s.overlay))
 	}
 	return nil
 }