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_line_editor/test_line_editor.cpp

@@ -0,0 +1,104 @@
+#include <unity.h>
+
+#include "line_editor.h"
+
+using namespace roro;
+
+void setUp() {}
+void tearDown() {}
+
+void test_typing_appends_at_cursor() {
+    LineEditor e(32);
+    e.insert('h');
+    e.insert('i');
+    TEST_ASSERT_EQUAL_STRING("hi", e.text().c_str());
+    TEST_ASSERT_EQUAL(2, e.cursor());
+}
+
+void test_accented_characters_are_stored_as_utf8() {
+    LineEditor e(32);
+    e.insert(0xE9);  // é
+    TEST_ASSERT_EQUAL_STRING("\xC3\xA9", e.text().c_str());
+    TEST_ASSERT_EQUAL(2, e.cursor());
+}
+
+void test_cursor_moves_over_whole_characters() {
+    LineEditor e(32);
+    e.setText("a\xC3\xA9z");  // aéz
+    TEST_ASSERT_EQUAL(4, e.cursor());
+    e.left();
+    TEST_ASSERT_EQUAL(3, e.cursor());
+    e.left();
+    TEST_ASSERT_EQUAL(1, e.cursor());  // skipped both bytes of é
+    e.right();
+    TEST_ASSERT_EQUAL(3, e.cursor());
+}
+
+void test_cursor_stops_at_both_ends() {
+    LineEditor e(32);
+    e.setText("ab");
+    e.right();
+    TEST_ASSERT_EQUAL(2, e.cursor());
+    e.home();
+    e.left();
+    TEST_ASSERT_EQUAL(0, e.cursor());
+    e.end();
+    TEST_ASSERT_EQUAL(2, e.cursor());
+}
+
+void test_insert_in_the_middle() {
+    LineEditor e(32);
+    e.setText("ac");
+    e.left();
+    e.insert('b');
+    TEST_ASSERT_EQUAL_STRING("abc", e.text().c_str());
+    TEST_ASSERT_EQUAL(2, e.cursor());
+}
+
+void test_backspace_removes_the_whole_character_before_the_cursor() {
+    LineEditor e(32);
+    e.setText("a\xC3\xA9");
+    e.backspace();
+    TEST_ASSERT_EQUAL_STRING("a", e.text().c_str());
+    e.home();
+    e.backspace();
+    TEST_ASSERT_EQUAL_STRING("a", e.text().c_str());
+}
+
+void test_insert_rejected_when_it_would_exceed_max_bytes() {
+    LineEditor e(3);
+    e.setText("ab");
+    TEST_ASSERT_FALSE(e.insert(0xE9));  // 2 bytes, only 1 left
+    TEST_ASSERT_TRUE(e.insert('c'));
+    TEST_ASSERT_FALSE(e.insert('d'));
+    TEST_ASSERT_EQUAL_STRING("abc", e.text().c_str());
+}
+
+void test_set_text_truncates_on_a_character_boundary() {
+    LineEditor e(2);
+    e.setText("a\xC3\xA9");
+    TEST_ASSERT_EQUAL_STRING("a", e.text().c_str());
+}
+
+void test_three_and_four_byte_characters() {
+    LineEditor e(32);
+    e.insert(0x20AC);   // €
+    e.insert(0x1F600);  // 😀
+    TEST_ASSERT_EQUAL(7, e.text().size());
+    e.backspace();
+    TEST_ASSERT_EQUAL_STRING("\xE2\x82\xAC", e.text().c_str());
+}
+
+int main() {
+    UNITY_BEGIN();
+    RUN_TEST(test_typing_appends_at_cursor);
+    RUN_TEST(test_accented_characters_are_stored_as_utf8);
+    RUN_TEST(test_cursor_moves_over_whole_characters);
+    RUN_TEST(test_cursor_stops_at_both_ends);
+    RUN_TEST(test_insert_in_the_middle);
+    RUN_TEST(test_backspace_removes_the_whole_character_before_the_cursor);
+    RUN_TEST(test_insert_rejected_when_it_would_exceed_max_bytes);
+    RUN_TEST(test_set_text_truncates_on_a_character_boundary);
+    RUN_TEST(test_three_and_four_byte_characters);
+    return UNITY_END();
+}