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/signals.go

@@ -586,9 +586,24 @@ func watchAdvisories(f *signalFacts) []conditions.Observation {
 		}
 		out = append(out, o)
 	}
+	if f.busFaultsErr != nil {
+		out = append(out, busFaultsUnread(f.busFaultsErr))
+	}
 	return append(out, watchDeadLetters(f)...)
 }
 
+// kindBusFaultsUnread is the bus module's events not read back (novox/hq ADR 0357): every other user's
+// slow consumer is unheard while it lasts, and the controller's own signals are still said.
+const kindBusFaultsUnread = "bus-faults-unread"
+
+func busFaultsUnread(err error) conditions.Observation {
+	return conditions.Observation{Scope: conditions.ScopeBus, ID: "bus-module", Kind: kindBusFaultsUnread,
+		Severity: conditions.Warning, Confirm: true,
+		Summary: "what the bus module said of its users' connections could not be read back, so a slow consumer " +
+			"or a connection the bus closed is not said until it can",
+		Said: firstLine(err.Error())}
+}
+
 // busUserClosed words an advisory about a bus user's connection the server closed (novox/hq ADR 0357): the
 // user as the operator says it and the machine it is on, the dotted user name kept in the evidence only.
 func busUserClosed(o *conditions.Observation, a link.Advisory, host string) {