loop, store: read a fact family by prefix (V-444)

A rule over a key set that only exists at read time cannot declare its keys
at wiring time. Kuma has one monitor per service and the names live in the
gauge, so the rule declares a prefix and the gatherer resolves the family per
tick.

ServiceDownRule now fires on any monitor reading down, names it through
DownServices, and is edge-triggered: a service that stays down is one nudge,
not one per tick with cooldown as the only brake.

internal/loop/gather.go

@@ -91,6 +91,20 @@ func (g *Gatherer) GatherState(ctx context.Context, now time.Time) (State, []sto
 		return State{}, nil, err
 	}
 
+	// prefix families — the keys a rule cannot name at wiring time (one fact
+	// per kuma monitor). Loaded into the same map; State.FactsUnder reads them.
+	for _, r := range g.rules {
+		for _, p := range r.WantPrefixes {
+			fam, err := g.store.LatestFactsByPrefix(ctx, p)
+			if err != nil {
+				return State{}, nil, err
+			}
+			for _, f := range fam {
+				facts[f.Key] = f
+			}
+		}
+	}
+
 	// last nudge per rule + cooldown-until derived from the active cooldown.
 	// "active" = the feedback tuner's persisted base if one exists, else the
 	// rule's static Base. LatestFactBySource is the trust-by-provenance read