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

lib/ui/src/toast_queue.cpp

@@ -0,0 +1,40 @@
+#include "toast_queue.h"
+
+namespace roro {
+
+void ToastQueue::push(const std::string& text, NotificationLevel level, uint32_t nowMs) {
+    current(nowMs);  // retire an expired Toast first, so the new one gets its full time
+    if (queue_.size() == capacity_) {
+        // Keep the visible one; drop the oldest waiting.
+        queue_.erase(queue_.begin() + (showing_ && queue_.size() > 1 ? 1 : 0));
+        if (!showing_) changed_ = true;
+    }
+    queue_.push_back({text, level});
+    if (!showing_) {
+        showing_ = true;
+        shownAtMs_ = nowMs;
+        changed_ = true;
+    }
+}
+
+const Toast* ToastQueue::current(uint32_t nowMs) {
+    if (!showing_) return nullptr;
+    if (nowMs - shownAtMs_ >= durationMs_) dismiss(shownAtMs_ + durationMs_);
+    return showing_ ? &queue_.front() : nullptr;
+}
+
+void ToastQueue::dismiss(uint32_t nowMs) {
+    if (!showing_) return;
+    queue_.pop_front();
+    changed_ = true;
+    showing_ = !queue_.empty();
+    shownAtMs_ = nowMs;
+}
+
+bool ToastQueue::takeChanged() {
+    bool c = changed_;
+    changed_ = false;
+    return c;
+}
+
+}  // namespace roro