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/state.go

@@ -23,6 +23,8 @@
 package loop
 
 import (
+	"sort"
+	"strings"
 	"time"
 
 	"github.com/kami/maven/internal/store"
@@ -95,6 +97,32 @@ func (s State) Fact(key string) (store.Fact, bool) {
 	return f, true
 }
 
+// FactsUnder returns every gathered fact whose key starts with prefix, ordered
+// by key so a caller that names them speaks them in a stable order. Facts with
+// a zero Ts are skipped, the same "no data" rule Fact applies.
+func (s State) FactsUnder(prefix string) []store.Fact {
+	var out []store.Fact
+	for k, f := range s.Facts {
+		if strings.HasPrefix(k, prefix) && !f.Ts.IsZero() {
+			out = append(out, f)
+		}
+	}
+	sort.Slice(out, func(i, j int) bool { return out[i].Key < out[j].Key })
+	return out
+}
+
+// NudgedSince reports whether rule already sent a nudge at or after ts.
+//
+// It is what makes a rule edge-triggered. A polled fact is written only when
+// the value changes, so its Ts is the moment the service went down — but the
+// predicate reads the current value, so a service that stays down keeps
+// qualifying forever and cooldown alone only slows the repetition. Asking
+// whether he was already told about THIS transition stops it.
+func (s State) NudgedSince(rule string, ts time.Time) bool {
+	n, ok := s.LastNudge[rule]
+	return ok && !n.Ts.Before(ts)
+}
+
 // Since returns the duration since the latest fact for key, or (0,false).
 // "false" ⇒ no data ⇒ shuts up when uncertain.
 func (s State) Since(key string) (time.Duration, bool) {