Wire real PS/2 mouse packets through to input events

Replace the mouse driver's synthetic stream with the real path, the
way the keyboard was wired:

- The auxiliary port's IRQ12 is enumerated on the mouse's own ACPI
  node (PNP0F13), and the kernel only lets a device's claimer bind or
  ack its IRQs — so ps2-bus now claims that node alongside the
  controller whenever port 2 carries a device, binds IRQ12 to its one
  endpoint, and re-arms whichever line the notification's badge names.
  The forwarding loop already routed auxiliary bytes by status bit 5.
- mouse-packet.zig (new, pure, host-tested): three-byte stream-mode
  packet assembly — bit-3 sync with resynchronization, ACK/BAT bytes
  dropped at packet start, nine-bit two's-complement movement,
  overflow packets discarded, and PS/2 positive-Y-up converted to the
  screen convention (positive down).
- mouse.zig mirrors the keyboard driver: no hardware claim, attaches
  to the bus as its mouse, and publishes button_down/button_up per
  changed button plus motion events with the pressed-button mask.

Verified end to end in QEMU via monitor mouse_move/mouse_button:
motion round-trips in screen coordinates, buttons transition with the
right mask, and keyboard events keep flowing alongside. The follow-up
is the IntelliMouse magic-knock for a scroll wheel (four-byte packets)
and scroll events.

build.zig

@@ -511,6 +511,7 @@ pub fn build(b: *std.Build) void {
         "system/devices/acpi-ids.zig", // _HID name decoding
         "library/mmio/mmio.zig", // barriers assemble + registers round-trip
         "system/drivers/ps2-bus/scancode.zig", // set-2 decode + keyboard state machine
+        "system/drivers/ps2-bus/mouse-packet.zig", // 3-byte mouse packet assembly
     }) |root| {
         const mod_tests = b.addTest(.{
             .root_module = b.createModule(.{