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).

system/services/input-source/input-source.zig

@@ -0,0 +1,33 @@
+//! system/services/input-source — a hardware-free synthetic keyboard source, used to
+//! exercise the input service end to end without a real PS/2 controller (the `input` test
+//! case, and any bring-up where there is no keyboard). It stands in for a driver: it
+//! connects to the input service and `publish`es a rolling stream of key events, which the
+//! service broadcasts to every subscriber.
+//!
+//! It stays silent after startup (no per-event logging) so it can share the boot serial
+//! transcript with a subscriber whose output is the test's success marker. The real
+//! keyboard driver publishes the same synthetic stream today; swapping in decoded
+//! scancodes is a follow-up (see docs/input.md).
+
+const runtime = @import("runtime");
+const input = runtime.input;
+const system = runtime.system;
+
+pub fn main() void {
+    var source = input.connectSource() orelse {
+        _ = system.write("input-source: input service unavailable\n");
+        return;
+    };
+    _ = system.write("input-source: publishing synthetic key events\n");
+
+    var step: usize = 0;
+    while (true) : (step +%= 1) {
+        _ = source.publish(input.syntheticEvent(step));
+        system.sleep(200);
+    }
+}
+
+pub const panic = runtime.panic;
+comptime {
+    _ = &runtime.start._start;
+}