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/kernel/tests.zig

@@ -1587,13 +1587,14 @@ fn vfsTest(boot_information: *const BootInformation) void {
     result();
 }
 
-/// The full input path: spawn the input service, a synthetic keyboard source, and a
-/// subscriber from the initial_ramdisk. The source publishes key events; the service
-/// broadcasts them (with the asynchronous ipc_send); the subscriber receives them and —
-/// only once it has — heartbeats "input-test: ok". Seeing that marker proves an event
-/// travelled source -> service -> subscriber over IPC, exercising the async buffered-send
-/// primitive and capability-passing subscription. The source and service stay silent
-/// after startup so the subscriber's line is the one left in the shared evidence buffer.
+/// The full input path: spawn the input service, a synthetic source, and a subscriber from
+/// the initial_ramdisk. The source publishes keyboard, mouse, and joystick events in turn;
+/// the service routes them (with the asynchronous ipc_send) to the subscriber, which took
+/// all three classes and — only once it has received one — heartbeats "input-test: ok".
+/// Seeing that marker proves an event travelled source -> service -> subscriber over IPC,
+/// exercising the async buffered-send primitive, capability-passing subscription, and
+/// per-device routing. The source and service stay silent after startup so the subscriber's
+/// line is the one left in the shared evidence buffer.
 fn inputTest(boot_information: *const BootInformation) void {
     log("DANOS-TEST-BEGIN: input\n", .{});
     if (boot_information.initial_ramdisk_len == 0) {