fix(schema): a non-final Schema subclass built after its parent leaks internal keys from to_dict() #1498

Describe the bug
#1490 for Schema: once a non-final Schema parent has been instantiated, its non-final children are never finalised. Their to_dict() then leaks internal names.

Reproduction

from pcapkit.protocols.schema.schema import Schema
from pcapkit.corekit.fields.numbers import UInt8Field
class P(Schema):
    a: 'int' = UInt8Field()
class C(P):
    b: 'int' = UInt8Field()
P.unpack(b'\x01')
list(C.unpack(b'\x01\x02').to_dict())   # ['__map__', '__map_reverse__', '__buffer__', '__updated__', 'a', 'b']

len() and iteration are correct (2, ['a', 'b']). C.__dict__ has no __finalised__, and C.__init__ is P.__init__.

Expected behavior
to_dict() returns ['a', 'b'], whichever class was built first.

System information
Linux, CPython 3.14.7, main at 00aa1fc90.

Additional context

• Cause: Schema.__new__ checks the inherited state, if cls.__finalised__ == FinalisedState.NONE: (pcapkit/protocols/schema/schema.py:472). fix(corekit): finalise each non-final Info subclass on its own state (#1490) #1496 fixes the same check in Info by reading the class's own __dict__.
• Found by: the fix(corekit): finalise each non-final Info subclass on its own state (#1490) #1496 cross-review.