Add widget kit, input layer with Compose Key, Launcher and demo apps

Steps 5 and 6 of M0.
- lib/ui (host-tested): ListModel, LineEditor (UTF-8, byte-limited),
  wrapText, ToastQueue, DialogModel
- lib/input (host-tested): KeyMapper turns keyboard state into logical
  keys (Fn arrows, ` Back, Fn+` Home) and composes accents with opt
- src/ui: off-screen 16-bit frame pushed in one transfer (fixes the
  flicker), Status Bar, list, text, line editor, dialog, Toast widgets,
  Notifier (Toast + beep + LED), vendored public-domain Latin-1 fonts
- Apps: minimal Launcher, Widget demo, Diagnostics (SD erase now
  behind a confirmation dialog)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

test/test_app_manager/test_app_manager.cpp

@@ -18,7 +18,9 @@ struct FakeApp : App {
         keys.push_back(e);
         return handles;
     }
+    void update(uint32_t nowMs) override { lastUpdate = nowMs; }
     void draw(Canvas&) override {}
+    uint32_t lastUpdate = 0;
     std::vector<KeyEvent> keys;
     bool handles = false;
 
@@ -145,6 +147,14 @@ void test_app_can_request_its_own_redraw() {
     TEST_ASSERT_FALSE(f.manager.takeRedraw());
 }
 
+void test_update_reaches_only_the_foreground_app() {
+    Fixture f;
+    f.manager.open("notes");
+    f.manager.update(42);
+    TEST_ASSERT_EQUAL(42, f.notes.lastUpdate);
+    TEST_ASSERT_EQUAL(0, f.launcher.lastUpdate);
+}
+
 int main() {
     UNITY_BEGIN();
     RUN_TEST(test_launcher_is_in_foreground_after_begin);
@@ -159,5 +169,6 @@ int main() {
     RUN_TEST(test_visible_apps_exclude_hidden_ones_and_keep_registration_order);
     RUN_TEST(test_switching_apps_requests_a_redraw);
     RUN_TEST(test_app_can_request_its_own_redraw);
+    RUN_TEST(test_update_reaches_only_the_foreground_app);
     return UNITY_END();
 }