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/service_manager.h

@@ -0,0 +1,31 @@
+#pragma once
+
+#include <vector>
+
+#include "service.h"
+
+namespace roro {
+
+class ServiceManager {
+   public:
+    void add(Service& service);
+
+    // Starts services in the order added; each gets its first tick on the next tick() call.
+    void startAll(uint32_t nowMs);
+    // Stops services in reverse order.
+    void stopAll();
+
+    // Ticks every service whose interval has elapsed. Missed intervals are not replayed.
+    void tick(uint32_t nowMs);
+
+   private:
+    struct Entry {
+        Service* service;
+        uint32_t lastTickMs;
+        bool due;
+    };
+    std::vector<Entry> entries_;
+    bool running_ = false;
+};
+
+}  // namespace roro