The application surface: enumerate, subscribe, and device-list (M18.3)

Applications ask the device manager for the tree (enumerate: a header
plus ChildEntry records) and subscribe to published add/remove events by
handing their endpoint over as the call's capability — the input-service
pattern; events are the same ChildAdded/ChildRemoved structs the bus
drivers send, one encoding in both directions. device-list is the first
client: it prints the tree, subscribes, and narrates the events through
a driver restart. The protocol's message maximum is capped at the
kernel's IPC MESSAGE_MAXIMUM (256 bytes, ten entries per reply; paging
joins the protocol when a tree outgrows one message). The startUserTask
debug print is gone: it wrote to serial unserialized against user-space
lines and sheared concurrent log markers in half — the root cause of the
scenario flakes.

library/runtime/service.zig

@@ -22,8 +22,10 @@ pub const Callbacks = struct {
     /// Return false to abort startup (the process exits).
     init: ?*const fn (endpoint: ipc.Handle) bool = null,
     /// One protocol request from `sender` (a task id): write the reply into
-    /// `reply`, return its length. The zero-length ping never reaches this.
-    on_message: *const fn (message: []const u8, reply: []u8, sender: u32) usize,
+    /// `reply`, return its length. `capability` is the handle the request
+    /// carried, if any (M13 cap passing — how a subscriber hands over its
+    /// endpoint). The zero-length ping never reaches this.
+    on_message: *const fn (message: []const u8, reply: []u8, sender: u32, capability: ?ipc.Handle) usize,
     /// A notification that is not a signal — a subscribed exit event, a bound
     /// IRQ, a timer landing. The raw badge; decode with the ipc helpers.
     on_notification: ?*const fn (badge: u64) void = null,
@@ -76,6 +78,6 @@ pub fn run(comptime maximum_message: usize, callbacks: Callbacks) void {
             reply_len = 0; // the universal ping: a zero-length reply, from the harness
             continue;
         }
-        reply_len = callbacks.on_message(receive[0..got.len], &reply_buffer, got.senderTaskId());
+        reply_len = callbacks.on_message(receive[0..got.len], &reply_buffer, got.senderTaskId(), got.cap);
     }
 }