std::machines::toggleMinimal cross-module Moore machine fixture.
Used by examples/machine/run_cross_module_toggle.hew to exercise the
HIR cross-module machine-binding path: a bare Flip event and qualified
Toggle::Off / Toggle::On state ctors referenced from a different
module must resolve through machine_ctor_registry after the HIR
pre-pass walks program.module_graph (see hew-hir/src/lower.rs).
Intentionally tiny — Unit states only, no entry/exit blocks, no
transition payloads — so it exercises only the substrate path under
test without depending on MIR-layer machinery (struct-state ctors,
event.handle field access, state wildcard transitions) that is
out of scope here.