Drops the polymorphic XuiActionPayload variant and the per-pad
setXuiServerAction/getXuiServerAction polling on IGameServices in
favour of a std::variant of typed action structs in
minecraft/server/ServerAction.h. MinecraftServer owns the queue,
drains it from the tick loop via std::visit, and exposes
queueServerAction() that any thread can call. Eliminates the
dynamic_cast across the minecraft<-app boundary, the per-pad slot,
and two busy-wait-for-Idle loops.
