Generalize input module to mouse and joystick/gamepad events

Extend the input service beyond the keyboard so mouse and joystick/gamepad
drivers can broadcast too, with per-device publish and subscribe methods.

- protocol: KeyEvent joins MouseEvent (motion/buttons/scroll) and
  JoystickEvent (axes/buttons), all carried in a common InputEvent envelope
  tagged with a DeviceKind. A subscribe request carries a device_mask, so a
  subscriber names the classes it wants and the service routes each event only
  to interested subscribers (a mouse-only listener never wakes for keystrokes).
- runtime: per-device publish methods (publishKeyboardEvent/publishMouseEvent/
  publishJoystickEvent) and subscribe helpers (subscribeKeyboard/Mouse/Joystick,
  each typed, plus subscribe(mask)/subscribeAll returning the tagged envelope).
- service: subscriber table gains a device_mask; broadcast routes by the
  event's device class.
- mouse driver now publishes (synthetic) mouse events like the keyboard driver;
  input-source cycles all three classes; input-test subscribes to all and only
  emits its "ok" marker once it has received one of each class — so the passing
  test proves per-device routing, not just delivery. Real HID decoding stays a
  follow-up.

No kernel changes: ipc_send is generic and the 36-byte InputEvent fits its
64-byte payload. Full QEMU suite 48/48; serial log confirms keyboard, mouse,
and joystick all reach one subscription.

system/drivers/ps2-bus/keyboard.zig

@@ -48,7 +48,7 @@ pub fn main() void {
 
     var step: usize = 0;
     while (true) : (step +%= 1) {
-        _ = source.publish(runtime.input.syntheticEvent(step));
+        _ = source.publishKeyboardEvent(runtime.input.syntheticKeyEvent(step));
         runtime.system.sleep(200);
     }
 }