Serve the read verbs on the serving controller's own connection, and name every connection

Each verb ran as a process that dialled the bus, so hundreds of short
connections an hour, all named mesh-controller, hid any client reconnecting
in a loop (hq issue 327). D15 now says a user whose connections keep dropping.

cmd/mesh-controller/servingbus.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"fmt"
+	"sync/atomic"
+
+	"github.com/novox/mesh-controller/internal/broker"
+)
+
+// The serving controller's own connection, lent to whatever it does for a moment (novox/hq issue 327).
+//
+// Every place that needed the bus for a moment dialled it: a verb's own process, and in the serving
+// controller a watchdog tick reading whether the build seat paused, a walk's step reading readiness, a
+// queue read. Each paid a connection, a TLS handshake and a login on the control node, and hundreds an
+// hour hid in the server's connection total the one thing it would show: a client reconnecting in a loop.
+// The serving controller is on the bus already; what it does is done on that connection.
+
+// servingBus is the serving controller's connection, set when it starts serving; nil in every other
+// process, which dials its own.
+var servingBus atomic.Pointer[broker.JetStream]
+
+// aBus is a connection for something done for a moment: the serving controller's own, lent — so its
+// Close closes nothing — when this process is it, and otherwise one dialled for it, named for this
+// process (broker.ConnectionName), which its Close closes.
+func aBus() (*broker.JetStream, error) {
+	if serving := servingBus.Load(); serving != nil {
+		return broker.Borrow(serving), nil
+	}
+	address, err := broker.BusAddress()
+	if err != nil {
+		return nil, err
+	}
+	js, err := broker.Dial(address)
+	if err != nil {
+		return nil, fmt.Errorf("cannot reach the bus: %w", err)
+	}
+	return js, nil
+}