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).

library/runtime/ipc.zig

@@ -79,6 +79,16 @@ pub fn call(h: Handle, message: []const u8, reply: []u8) CallError!usize {
     return (try callCap(h, message, reply, null)).len;
 }
 
+/// Post `message` to endpoint `h`'s asynchronous queue and return immediately — no
+/// rendezvous, no reply, no blocking. The receiver picks it up through `replyWait` as a
+/// buffered message (`Received.isMessage`). Unlike `call`, this **cannot hang on a dead
+/// or slow peer**, which is why a broadcaster (the input service) delivers events this
+/// way. The payload must fit an endpoint slot (64 bytes); a full queue drops the oldest
+/// message. Returns false on failure (bad handle, oversized payload, bad buffer).
+pub fn send(h: Handle, message: []const u8) bool {
+    return !failed(sc.systemCall3(.ipc_send, h, @intFromPtr(message.ptr), message.len));
+}
+
 /// Set in `Received.badge` when what arrived is an asynchronous notification — a
 /// bound device interrupt — rather than a client's message. The low bits carry the
 /// GSI. See `isNotification`.
@@ -89,6 +99,12 @@ pub const notify_badge_bit: u64 = abi.notify_badge_bit;
 /// rather than a device interrupt. The low bits carry the child's process id.
 pub const notify_exit_bit: u64 = abi.notify_exit_bit;
 
+/// Set alongside `notify_badge_bit` when the wake-up is a **buffered message** — a payload
+/// posted with `send` (`ipc_send`) — rather than a bare device interrupt or child-exit
+/// notice. The payload is in the `replyWait` receive buffer (`Received.len` bytes); the
+/// low bits of the badge carry the sender's task id. See `Received.isMessage`.
+pub const notify_message_bit: u64 = abi.notify_message_bit;
+
 /// The result of a `replyWait`: the request length, the sender's badge (a task id, or
 /// an IRQ notification if the high bit is set), and any capability the request carried.
 pub const Received = struct {
@@ -109,6 +125,19 @@ pub const Received = struct {
         return self.isNotification() and self.badge & notify_exit_bit != 0;
     }
 
+    /// True if this wake-up is a **buffered message** posted with `send` (`ipc_send`):
+    /// there is a payload in the receive buffer (`self.len` bytes) and no reply is owed.
+    /// The subscriber side of a broadcast branches on this.
+    pub fn isMessage(self: Received) bool {
+        return self.isNotification() and self.badge & notify_message_bit != 0;
+    }
+
+    /// The task id of whoever posted a buffered message, meaningful only when
+    /// `isMessage`. (The badge's low bits, with the three high marker bits masked off.)
+    pub fn senderTaskId(self: Received) u32 {
+        return @intCast(self.badge & ~(notify_badge_bit | notify_exit_bit | notify_message_bit));
+    }
+
     /// The interrupt source (a GSI), meaningful only when `isNotification` and
     /// not `isChildExit`.
     pub fn source(self: Received) u64 {