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.

system/services/process-test/process-test.zig

@@ -51,8 +51,9 @@ fn awaitChildExit(endpoint: runtime.ipc.Handle) u32 {
 /// The harness-run child of the signals test: echoes requests, logs the two
 /// signals it handles. Terminate makes run() return, and returning from main is
 /// the clean exit the parent reads as ExitReason.exited.
-fn echo(message: []const u8, reply: []u8, sender: u32) usize {
+fn echo(message: []const u8, reply: []u8, sender: u32, capability: ?runtime.ipc.Handle) usize {
     _ = sender;
+    _ = capability;
     const n = @min(message.len, reply.len);
     @memcpy(reply[0..n], message[0..n]);
     return n;