Keep neoirc-cli responding when a key or a typed line updates the screen (closes #223)

The Enter handler of the input line and the Alt and digit key handler
ran the typed line on the tview event loop. Every UI method that
changes the screen waits for that loop, so the client stopped
responding. Each line now runs on its own goroutine, which first waits
for the line before it, so lines keep the order they were typed in.

/window now runs off the event loop. It reads the buffer name through
the new BufferName, which runs on the loop, and switches only when
that buffer exists.

One new test drives a running UI on a simulation screen with Alt and
1, then a typed line and Enter. Another holds the first line and
checks that the second waits for it.

Model: opus-5-5

internal/cli/app.go

@@ -610,21 +610,24 @@ func (a *App) cmdWindow(args string) {
 
 	_, _ = fmt.Sscanf(args, "%d", &bufIndex)
 
-	a.ui.SwitchBuffer(bufIndex)
-
 	a.mu.Lock()
 	nick := a.nick
 	a.mu.Unlock()
 
-	if bufIndex >= 0 && bufIndex < a.ui.BufferCount() {
-		buf := a.ui.buffers[bufIndex]
-		if buf.Name != "(status)" {
+	// Read the name before switching. A message from the server can
+	// add a buffer at any time, but only at the end, so bufIndex
+	// still names this buffer when SwitchBuffer runs.
+	name, found := a.ui.BufferName(bufIndex)
+	if found {
+		a.ui.SwitchBuffer(bufIndex)
+
+		if name != "(status)" {
 			a.mu.Lock()
-			a.target = buf.Name
+			a.target = name
 			a.mu.Unlock()
 
 			a.ui.SetStatus(
-				nick, buf.Name, "connected",
+				nick, name, "connected",
 			)
 		} else {
 			a.mu.Lock()