Subclassing a StateChart with callbacks in a State.Compound body raises AttributeError #646

Subclassing any StateChart whose compound state body contains a callback raises
AttributeError: 'State' object has no attribute '_callbacks' at subclass creation time.

Reproduction

from statemachine import State, StateChart
class Journey(StateChart):
 class shire(State.Compound):
 bag_end = State(initial=True)
 green_dragon = State()
 visit_pub = bag_end.to(green_dragon)
 def on_enter_green_dragon(self):
 ...
 road = State(final=True)
 depart = shire.to(road)
class Child(Journey): # AttributeError
 pass

Traceback (most recent call last):
 ...
 File "statemachine/factory.py", line 168, in _unpack_builders_callbacks
 if state._callbacks:
 ^^^^^^^^^^^^^^^^
AttributeError: 'State' object has no attribute '_callbacks'

Cause

NestedStateFactory builds the compound as a State instance carrying _callbacks, and
StateMachineMetaclass._unpack_builders_callbacks consumes it with del state._callbacks:

def _unpack_builders_callbacks(cls):
 callbacks = {}
 for state in iterate_states(cls.states):
 if state._callbacks:
 callbacks.update(state._callbacks)
 del state._callbacks

add_inherited reuses the same State objects in the subclass, so the second metaclass run
re-reads an attribute the first run deleted. Any callback in the body triggers it, including a
plain method or an on=/cond= decorator.

Notes

Only compound bodies are affected: a flat StateChart never populates _callbacks on a state.
A compound body with no callbacks at all also subclasses fine, since _callbacks stays None
and the attribute is never deleted.

Setting state._callbacks = None instead of deleting it, or guarding with
getattr(state, "_callbacks", None), both fix the crash. The subclass does not need the
setattr again, since it inherits the attributes the base class already received.

Reproduced on the current develop.