Admit a first lease when a quota limit has no receipt history

Burn-in run 2 ingested its task and then sat queued forever. Every herdr in
the live config declares quota_limit_5h and quota_limit_weekly, the event log
holds zero QuotaReported events, and QuotaSince reported an empty window as
unknown. QuotaAvailability fails closed on unknown, so no harness could ever
be leased, and the only producer of a receipt is a completed lease.

The event log is Orchestra's whole accounting source, so a window holding no
receipts is observable zero consumption. QuotaSince now reports known for an
empty window and for a harness that has never reported. A receipt that
declares its own consumption unknown still fails closed.

The refusal also lied about its cause. federatedAvailability collapsed a base
gate refusal into the federation health string, so router health said "stale
heartbeat or unhealthy local backend" while the heartbeat was one second old.
Availability gates now name themselves through an optional
ReasonedAvailability contract: quota refusals say whether usage is unknown or
the window is exhausted and by how much, and worker refusals distinguish an
unregistered worker, a never-probed backend, a stale heartbeat, a stale
health check, and an unreachable backend.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/store_test.go

@@ -637,3 +637,29 @@ func TestReleaseTransactionSurvivesReLeaseUntilMatchingPickup(t *testing.T) {
 		t.Fatalf("pickup not bound to transaction/epoch: %+v", task)
 	}
 }
+
+// An empty window is observable zero consumption, not missing data. Reporting
+// it as unknown made the router refuse every harness that had a configured
+// quota limit and no receipt history, which no first lease could ever produce.
+func TestQuotaSinceReportsEmptyWindowAsKnownZero(t *testing.T) {
+	s, err := Open(t.TempDir())
+	if err != nil {
+		t.Fatal(err)
+	}
+	now := time.Now().UTC()
+	if used, known := s.QuotaSince("fresh", now.Add(-fiveHours)); used != 0 || !known {
+		t.Fatalf("never-reported harness quota=(%v,%v), want (0,true)", used, known)
+	}
+	payload, _ := json.Marshal(map[string]any{"harness_id": "h1", "consumed": 7.0, "known": true})
+	if err := s.Append(domain.Event{ID: domain.NewID(), Type: "QuotaReported", TaskID: "quota", Version: 1, At: now.Add(-24 * time.Hour), Payload: payload, Surface: string(authz.System)}); err != nil {
+		t.Fatal(err)
+	}
+	if used, known := s.QuotaSince("h1", now.Add(-fiveHours)); used != 0 || !known {
+		t.Fatalf("receipts only outside the window quota=(%v,%v), want (0,true)", used, known)
+	}
+	if used, known := s.QuotaSince("h1", now.Add(-48*time.Hour)); used != 7 || !known {
+		t.Fatalf("receipts inside the window quota=(%v,%v), want (7,true)", used, known)
+	}
+}
+
+const fiveHours = 5 * time.Hour