The headless server added the multiplayer-fps game scene as a child node
(/root/HeadlessServer/multiplayer-fps/...), causing RPC node paths to
include the HeadlessServer/ prefix. Clients expected paths relative to
root (/root/multiplayer-fps/...), so every RPC would fail with
'Node not found'.
Fix: add the game scene directly to /root/ via call_deferred, making it
a sibling of the HeadlessServer node rather than a child. RPC paths now
match between server and client.
