Single player only runs on its server thread now. --in-frame and
--no-server-thread print a notice, and HORIZON_SERVER_THREAD is ignored.
This deletes the code that only the in-frame path reached: the window's
world loader and local player tick, its portal travel, the loading screen
and ring-by-ring streaming, and the in-frame LAN host (the host player in
the hub, its models of the guests, the reserved uuid). The world-upgrade
progress screen with its Cancel button moves to the server-thread start.
Tools, the harnesses and the docs no longer use the in-frame path.
Closes#6473
Co-Authored-By: Claude Opus 5.5 <[email protected]>
