Bound the read of bus-module events, keep S9 seeing when it fails, count the controller's own slow consumer once (hq issue 270, review of #326)

A storm of closures must not spend a watchdog tick, so the read stops at 2000
events and says the count is a floor. A failed read is its own condition
rather than blinding S9, so the controller's own signals are still said. The
controller's fall behind, told to its client and closed by the server, is one
fault, counted once.

cmd/mesh-controller/bus_users_closed_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"strings"
 	"testing"
 	"time"
@@ -67,3 +68,25 @@ func TestAUserTheBusClosedAsASlowConsumerIsOneConditionNamingItsModuleAndMachine
 		}
 	}
 }
+
+// **A read of the bus module's events that fails does not blind S9** (review of #326): the controller's own
+// signals are still said, and the failed read is a condition of its own.
+func TestABusModuleReadThatFailsKeepsTheControllersOwnSignals(t *testing.T) {
+	at := time.Date(2026, 10, 14, 12, 0, 0, 0, time.UTC)
+	f := &signalFacts{now: at.Add(time.Minute), host: "anchor",
+		advisories: []link.Advisory{{Kind: link.AdvisorySlowConsumer, ID: "controller", Said: "the controller fell behind",
+			First: at, Last: at, Count: 1}},
+		busFaultsErr: errors.New("nats: timeout")}
+	keys := map[string]bool{}
+	for _, o := range watchAdvisories(f) {
+		keys[o.Key()] = true
+	}
+	if !keys["bus.controller.slow-consumer"] || !keys["bus.bus-module.bus-faults-unread"] {
+		t.Fatalf("observed %v: the controller's own slow consumer and the unread events, both", keys)
+	}
+	for _, row := range signalsTable {
+		if row.Row == "S9" && row.needs(f) != nil {
+			t.Fatal("the S9 row went blind for a read of the bus module's events")
+		}
+	}
+}