Subclassing a StateChart whose nested body has callbacks raises AttributeError on _callbacks #658

Measured on develop:

class Base(StateChart):
    class c(State.Compound):
        a = State(initial=True)
        b = State(final=True)
        go = a.to(b)

        def on_enter_b(self):
            pass

class Sub(Base):
    pass

# AttributeError: 'State' object has no attribute '_callbacks'

add_inherited reuses the same State objects from the base class (cls.add_state(state.id, state)
over base.states), while _unpack_builders_callbacks consumes them destructively:

for state in iterate_states(cls.states):
    if state._callbacks:
        callbacks.update(state._callbacks)
        del state._callbacks

So the base class deletes the attribute, and the subclass reads it on the shared object and
crashes. It only triggers when a nested body actually declares a callback, which is why it has
gone unnoticed.

The _callbacks bag exists because NestedStateFactory has no owning class to bind to when the
nested body is evaluated, so it hands everything back for the metaclass to flatten afterwards.
#657 removes that bag entirely, which removes this failure along with it. A narrower fix is to
stop mutating shared state: read _callbacks without deleting it, or give the subclass its own
copy of the inherited State.

Found by an adversarial review of #645, unrelated to that change.

When fixing: add a subclassing example to docs/statecharts.md or docs/models.md if none
covers inheritance with nested bodies, and add an entry to the open release notes.