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/mouse.zig

@@ -40,8 +40,21 @@ pub fn main() void {
     }
     writeLine("system/drivers/ps2-bus/mouse: claimed device for hid {s}\n", .{hid});
 
+    // Broadcast mouse events through the input service so programs can listen for them
+    // (docs/input.md). As with the keyboard, decoding real PS/2 mouse packets is a
+    // follow-up; for now we publish the synthetic stand-in stream. The fan-out path is
+    // real, only the source of the movement is placeholder.
+    var source = runtime.input.connectSource() orelse {
+        _ = runtime.system.write("system/drivers/ps2-bus/mouse: input service unavailable\n");
+        return;
+    };
     _ = runtime.system.write("system/drivers/ps2-bus/mouse: ok\n");
-    while (true) runtime.system.sleep(1000);
+
+    var step: usize = 0;
+    while (true) : (step +%= 1) {
+        _ = source.publishMouseEvent(runtime.input.syntheticMouseEvent(step));
+        runtime.system.sleep(200);
+    }
 }
 
 pub const panic = runtime.panic;