MixinMinecraftServer

Server-lane heartbeat + the server-stop reap, as a mixin.

priority = Integer.MAX_VALUE: the heartbeat pumps at the tick's RETURN (postfix) so the eval sees the fully-settled post-tick server state, and max priority makes it fire as late as possible relative to other mods' RETURN injectors and keeps our injection alive if another mod overwrites tickServer (a lower-priority overwrite is applied before us).