Add aggregator backfill, pulling fine+coarse buckets from collectors

cmd/aggregator/cache.go

@@ -90,6 +90,26 @@ func (c *Cache) mergeFineBuckets(now time.Time) st.Snapshot {
 	return st.Snapshot{Timestamp: now, Entries: st.TopKFromMap(merged, st.CoarseTopK)}
 }
 
+// LoadHistorical pre-populates the ring buffers from backfill data before live
+// streaming begins. fine and coarse must be sorted oldest-first; each slice
+// must not exceed the respective ring size. Called once at startup, before Run.
+func (c *Cache) LoadHistorical(fine, coarse []st.Snapshot) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	for i, snap := range fine {
+		c.fineRing[i] = snap
+	}
+	c.fineFilled = len(fine)
+	c.fineHead = len(fine) % st.FineRingSize
+
+	for i, snap := range coarse {
+		c.coarseRing[i] = snap
+	}
+	c.coarseFilled = len(coarse)
+	c.coarseHead = len(coarse) % st.CoarseRingSize
+}
+
 // QueryTopN answers a TopN request from the ring buffers.
 func (c *Cache) QueryTopN(filter *pb.Filter, groupBy pb.GroupBy, n int, window pb.Window) []st.Entry {
 	cf := st.CompileFilter(filter)