Guard View.LinesHeight against a concurrent write

The count comes from the view's buffer, which a rendering task appends to on
its own goroutine, so reading it without the write mutex is a data race. Nobody
called it until now, which is why nothing has tripped over it; the next commit
does, from the UI thread while a render is still loading.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/gocui/view.go

@@ -2063,6 +2063,9 @@ func (v *View) ViewBufferLines() []string {
 
 // LinesHeight is the count of view lines (i.e. lines excluding wrapping)
 func (v *View) LinesHeight() int {
+	v.writeMutex.Lock()
+	defer v.writeMutex.Unlock()
+
 	return len(v.buf.lines)
 }