Add input module: broadcast keyboard events over IPC

Programs can now subscribe to keyboard events (key_down/key_up/key_press)
and drivers can broadcast them, through a new user-space input service.

The delivery model is forced by danos IPC: a synchronous rendezvous holds
one pending reply, so a server cannot park N subscribers blocked in a
"wait for next event" call — delivery must be push. But a synchronous push
has no timeout and the kernel never wakes a sender parked on a dead peer's
endpoint, so one dying subscriber would hang all input. So this lands the
roadmap's planned asynchronous buffered send and builds the service on it:

- ipc_send (syscall 26): non-blocking post to an endpoint's bounded payload
  ring, delivered through reply_wait as a buffered message (notify_message_bit).
  A full ring drops the oldest. It can never hang on a dead/slow peer.
- input-protocol + runtime.input helpers (subscribe/next, connectSource/
  publish) — the first real consumer of M13 capability passing: a subscriber
  hands the service its own endpoint as a capability.
- input service (fan-out via ipc_send, dead-subscriber pruning), a synthetic
  input-source, and input-test; the ps2-bus keyboard driver publishes to it.
  Real IRQ1 scancode decoding (which must live in the bus, the PNP0303 owner)
  is a documented follow-up; the source is synthetic for now.
- build/init wiring, an `input` QEMU case, and docs/input.md.

Full QEMU suite 48/48, including the new input case and every IPC/endpoint
regression (ipc, ipc-call, ipc-cap, vfs, hpet, bus, irqfree).

test/qemu_test.py

@@ -250,6 +250,12 @@ CASES = [
     {"name": "vfs",
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # The input service: a synthetic keyboard source publishes events, the service
+    # broadcasts them over the async ipc_send primitive, and a subscriber (which joined by
+    # passing its endpoint as a capability) receives them — source -> service -> subscriber.
+    {"name": "input",
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # IO passthrough + IRQ-as-IPC: a user-space HPET driver maps device MMIO into
     # its own address space, binds the device's interrupt to an IPC endpoint, and
     # is woken by the hardware five times while blocked (never polling).