feat(ui): setText on text nodes

Assigning .label repainted nothing until the caller also remembered to invalidate, and
forgetting produced a screen that silently stopped updating with no error to follow.
setText makes the pair atomic and skips the repaint when the text is unchanged, so an
app can push a value on every tick without comparing first.

test/ui_layout.lua

@@ -87,6 +87,17 @@ app:down(100, 20)
 app:up(100, 20)
 assert(fired == 1, "release inside must fire once")
 
+-- setText repaints only on a real change, so a caller may push a value every tick.
+local label = ui.text("12:00:00")
+local clockScreen = ui.screen(ui.box{label})
+clockScreen:draw()
+assert(not label.dirty, "a drawn node starts clean")
+label:setText("12:00:00")
+assert(not label.dirty, "unchanged text must not repaint")
+label:setText("12:00:01")
+assert(label.dirty, "changed text must repaint")
+assert(label.label == "12:00:01", label.label)
+
 -- The pressed look is held briefly, then cleared on a later draw.
 assert(button.pressed, "pressed look must outlast the release")
 device.now = device.now + 100