Pause spinner ticks when not refreshing.

Also, add a quick-and-dirty model for displaying basic performance
stats, currently just the number of calls to the root Update() and
View() methods.

tui/perf/perf.go

@@ -0,0 +1,42 @@
+package perf
+
+import (
+	"fmt"
+	"sync/atomic"
+
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+type Model struct {
+	updates *atomic.Int64
+	views   *atomic.Int64
+}
+
+func New() (m Model) {
+	m.updates = new(atomic.Int64)
+	m.views = new(atomic.Int64)
+	return
+}
+
+func (m Model) Init() tea.Cmd {
+	return nil
+}
+
+func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
+	return m, nil
+}
+
+func (m Model) View() string {
+	updates := m.updates.Load()
+	views := m.views.Load()
+	s := fmt.Sprintf("updates: %d\tviews: %d", updates, views)
+	return s
+}
+
+func (m Model) AddUpdate() {
+	m.updates.Add(1)
+}
+
+func (m Model) AddView() {
+	m.views.Add(1)
+}