#[machine]Expand description
Define a typed machine that carries durable context across states.
Apply #[machine] to a struct whose first generic parameter is the
#[state] enum family. Additional type and const generics are supported
after that state generic. Statum generates the typed machine surface,
builders, the machine-scoped machine::SomeState enum, a compatibility
alias machine::State = machine::SomeState, and helper items such as
machine::Fields for heterogeneous batch rebuilds.