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.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <vector>
+
+#include "app.h"
+
+namespace roro {
+
+struct AppInfo {
+    const char* id;
+    const char* title;
+    bool hidden;  // not listed in the Launcher, but can still be opened
+    App* app;
+};
+
+// Owns which App is in the foreground and routes keys. Home always returns to the Launcher;
+// Back is offered to the App first and returns to the Launcher if the App doesn't consume it.
+class AppManager {
+   public:
+    explicit AppManager(App& launcher);
+
+    void registerApp(const AppInfo& info);
+    std::vector<const AppInfo*> visibleApps() const;
+
+    void begin();
+    bool open(const char* id);
+    void home();
+
+    void handleKey(const KeyEvent& event);
+
+    App& foreground() const { return *foreground_; }
+
+    // True once after the screen needs redrawing (App switch, or the App asked for it).
+    bool takeRedraw();
+
+   private:
+    void switchTo(App& app);
+
+    App& launcher_;
+    App* foreground_;
+    std::vector<AppInfo> apps_;
+    bool redraw_ = true;
+};
+
+}  // namespace roro