Add core runtime: event bus, services, app lifecycle

lib/core is hardware-independent and unit-tested on the host:
- EventBus: fixed-size thread-safe queue, delivered on the UI task by
  dispatch(); drops newest when full and counts drops
- Service / ServiceManager: cooperative ticking at per-service intervals,
  no catch-up bursts, safe across millis() wraparound
- App / AppManager: one foreground App, Home always to Launcher, Back
  offered to the App first, hidden Apps, redraw requests
- KeyEvent: logical keys for the upcoming input layer

Firmware main loop now ticks services and dispatches events.

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

lib/core/src/app_manager.cpp

@@ -0,0 +1,59 @@
+#include "app_manager.h"
+
+#include <cstring>
+
+namespace roro {
+
+AppManager::AppManager(App& launcher) : launcher_(launcher), foreground_(&launcher) {}
+
+void AppManager::registerApp(const AppInfo& info) { apps_.push_back(info); }
+
+std::vector<const AppInfo*> AppManager::visibleApps() const {
+    std::vector<const AppInfo*> visible;
+    for (auto& info : apps_)
+        if (!info.hidden) visible.push_back(&info);
+    return visible;
+}
+
+void AppManager::begin() {
+    foreground_ = &launcher_;
+    launcher_.onEnter();
+    redraw_ = true;
+}
+
+bool AppManager::open(const char* id) {
+    for (auto& info : apps_) {
+        if (std::strcmp(info.id, id) == 0) {
+            switchTo(*info.app);
+            return true;
+        }
+    }
+    return false;
+}
+
+void AppManager::home() { switchTo(launcher_); }
+
+void AppManager::handleKey(const KeyEvent& event) {
+    if (event.key == Key::Home) {
+        home();
+        return;
+    }
+    bool consumed = foreground_->onKey(event);
+    if (!consumed && event.key == Key::Back) home();
+}
+
+bool AppManager::takeRedraw() {
+    bool r = redraw_ | foreground_->consumeRedraw();
+    redraw_ = false;
+    return r;
+}
+
+void AppManager::switchTo(App& app) {
+    if (&app == foreground_) return;
+    foreground_->onExit();
+    foreground_ = &app;
+    foreground_->onEnter();
+    redraw_ = true;
+}
+
+}  // namespace roro