fix(ui): 0.R4 - an action a UI button dispatches is reduced before the frame is presented

ludic.ui runs a frame's presses after drawing it, and what they dispatched waited for the end of the
phase - after the host had presented - so a button's change showed a frame late. Decided: drain, not a
phase per action. ui_show and ui_press drain the queue once the presses have run, so the code after
them and the frame presented next see the change; a host that runs presses some other way calls
drain_actions() before it presents.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/actions/ui_press_parts/Adder.ludic

@@ -0,0 +1,4 @@
+# Adder.ludic - a button that says what happened and changes nothing itself
+component Adder {
+  on add(n: int) { dispatch Scored { n: n } }
+}

examples/actions/ui_press_parts/Adder.xml

@@ -0,0 +1,3 @@
+<div>
+  <button on-click="add(5)">+5</button>
+</div>