Add threading model doc and fix server thread
Add THREADING.md describing the Rockbox cooperative scheduler,
yield contract, and rules for mixing Rockbox kernel threads with
Rust OS threads. Update server::start_server to spawn the HTTP
server and periodically call rb::system::sleep(rb::HZ) with a short
thread::sleep to yield the Rockbox scheduler instead of joining the
spawned handle. Include regenerated tonic/upnp code formatting tweaks.