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).

docs/README.md

@@ -52,7 +52,10 @@ rather than restate it. Roughly in the order things happen at runtime:
     microkernel's `ps`/`kill`/SIGCHLD: enumerate as a table snapshot, the
     supervision link as the kill authority, and child-exit notifications over the
     same endpoints IRQs arrive on.
-16. **[halting.md](halting.md) — halting.** Why a kernel can't just "exit", and
+16. **[input.md](input.md) — the input module.** Broadcasting keyboard events: why a
+    synchronous rendezvous can't fan out to many listeners, the asynchronous `ipc_send`
+    primitive built to fix it, and the subscribe/publish service layered on top.
+17. **[halting.md](halting.md) — halting.** Why a kernel can't just "exit", and
     how `while (true) hlt` parks the CPU safely once there's nothing left to do.
 
 Start with the north star: