Add headless test harness for simulated input sequences

TestHarness provides a builder-style API for scripting UI interactions
without a GPU: tap, drag, key, type_text, set_key_mods. It manages
layout, event processing, and state access in view-local coordinates.

13 tests covering: repeated taps, toggle, knob drag, two-view hstack
tap targeting, drag gesture state sequence, tap miss detection, key
events, type_text helper, keyboard modifiers, vertical list item
selection, cond state toggling, and tap action types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: wtholliday

src/lib.rs

@@ -64,6 +64,9 @@ mod snapshot_tests;
 #[cfg(test)]
 mod screenshot_tests;
 
+#[cfg(test)]
+mod test_harness;
+
 #[cfg(test)]
 mod tests {