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.

docs/input.md

@@ -126,11 +126,18 @@ the service delivers to its endpoint, which only the same thread could receive).
   control characters for Enter/Tab/Backspace/Escape, whose keysyms map to no Unicode. The
   layout defaults to `us`; the bus can pass another as the driver's argv[2] — the seam for
   a future settings source.
-- **The mouse is still synthetic.** [mouse.zig](../system/drivers/ps2-bus/mouse.zig)
-  publishes a placeholder motion/click stream; the hardware-free `input-source` likewise
-  rotates through all three classes (including a synthetic joystick, which has no driver
-  yet) via the `input.synthetic*Event` helpers. **Follow-up:** the mouse driver attaches to
-  the bus the way the keyboard does and decodes 3/4-byte packets into mouse events.
+- **The mouse is real too.** IRQ12 is enumerated on the auxiliary device's own ACPI node
+  (PNP0F13), so the bus claims that node alongside the controller and routes both IRQs to
+  its one endpoint, acking whichever line the notification's badge names.
+  [mouse.zig](../system/drivers/ps2-bus/mouse.zig) attaches the way the keyboard does and
+  assembles the forwarded bytes with
+  [mouse-packet.zig](../system/drivers/ps2-bus/mouse-packet.zig) (three-byte stream-mode
+  packets: sync/overflow handling, nine-bit movement, screen-convention `dy` — host-tested
+  under `zig build test`) into `button_down`/`button_up` transitions and `motion` events.
+  **Follow-up:** the IntelliMouse magic-knock for a scroll wheel (four-byte packets) and
+  `scroll` events. The hardware-free `input-source` still rotates through all three classes
+  synthetically (including a joystick, which has no driver yet) via the
+  `input.synthetic*Event` helpers.
 - **Drop-oldest under overflow** is a defined loss; the 16-slot ring absorbs normal bursts.
   Real backpressure/flow-control is future work.
 - **`publish` is unauthenticated** — any process may publish, consistent with the current