feat(harness): hold recovery while a Grafana alert is firing, add legacy:busy host resolution

キャンペーン第 2 回のグレーゾーン 3 件を受けた変更。SLO 3 系列に現れない症状
（レプリケーション一時停止、メモリ圧迫）でも、受信中のアラートが解決するまで
recovered を出さないよう、AlertRouter が firing 中のアラートを保持し judge が
alerts_ok を条件に加える。legacy:busy は直近 2 分で最も多くリクエストを受けた
MIG インスタンス（Mimir topk）を指し、run+シナリオ単位でキャッシュして revert が
同じインスタンスに当たるようにする（ILB のハッシュで api の接続が 1 台に偏る）。

harness/alerts.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"sort"
 	"sync"
 	"time"
 )
@@ -22,10 +23,37 @@ type AlertRouter struct {
 	app    *App
 	mu     sync.Mutex
 	timers map[string]*time.Timer // fingerprint -> escalation timer
+	firing map[string]string      // fingerprint -> alertname while Grafana reports it firing (judge: no recovery with an open alert)
 }
 
 func NewAlertRouter(a *App) *AlertRouter {
-	return &AlertRouter{app: a, timers: map[string]*time.Timer{}}
+	return &AlertRouter{app: a, timers: map[string]*time.Timer{}, firing: map[string]string{}}
+}
+
+// Firing lists the alert names currently firing (deduplicated, sorted).
+func (r *AlertRouter) Firing() []string {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	seen := map[string]bool{}
+	var out []string
+	for _, n := range r.firing {
+		if !seen[n] {
+			seen[n] = true
+			out = append(out, n)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
+func (r *AlertRouter) setFiring(fp, name string, on bool) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if on {
+		r.firing[fp] = name
+	} else {
+		delete(r.firing, fp)
+	}
 }
 
 // Grafana webhook payload (https://grafana.com/docs/grafana/latest/alerting/configure-notifications/manage-contact-points/integrations/webhook-notifier/)
@@ -80,6 +108,7 @@ func (r *AlertRouter) handleWebhook(w http.ResponseWriter, req *http.Request) {
 			"startsAt": al.StartsAt, "fingerprint": al.Fingerprint, "layer": al.Labels["layer"], "signature": sig}
 		r.app.store.Event(bg, runID, sid, "alert", "grafana", payload)
 		handled++
+		r.setFiring(al.Fingerprint, name, al.Status == "firing")
 		if al.Status != "firing" {
 			r.cancelTimer(al.Fingerprint)
 			continue