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/event.h

@@ -0,0 +1,41 @@
+#pragma once
+
+#include <cstdint>
+#include <cstring>
+
+namespace roro {
+
+// Everything Services announce to the UI. Add new kinds before Count.
+enum class EventType : uint8_t {
+    BatteryChanged,    // a = percent, b = millivolts
+    Notification,      // text = message, a = NotificationLevel
+    StorageThreshold,  // a = usage percent, b = threshold crossed (80 / 90 / 100)
+    SettingChanged,    // text = setting key
+    Count
+};
+
+// Small, fixed-size and copyable so it can cross tasks through a queue without allocating.
+struct Event {
+    static constexpr size_t kTextCapacity = 48;
+
+    EventType type;
+    int32_t a = 0;
+    int32_t b = 0;
+    char text[kTextCapacity] = {};
+
+    static Event withValues(EventType type, int32_t a, int32_t b = 0) {
+        Event e{type};
+        e.a = a;
+        e.b = b;
+        return e;
+    }
+
+    static Event withText(EventType type, const char* text, int32_t a = 0) {
+        Event e{type};
+        e.a = a;
+        std::strncpy(e.text, text, kTextCapacity - 1);
+        return e;
+    }
+};
+
+}  // namespace roro