Fixes #453.

A custom field serializer on a field of a flattened dataclass crashed serialization:

@serialize
class Child:
    value: int = field(serializer=str)

@serialize
class Parent:
    child: Child = field(flatten=True)

to_dict(Parent(child=Child(value=3)))  # SerdeError: name 'value_serializer' is not defined

Cause

A flattened dataclass field is rendered inline in the enclosing class's generated serialize function, so it emits references like value_serializer(...). But the loop that registers custom field serializers (and skip_if predicates) in the generated globals only iterated the top-level fields — the flattened child's serializer was never put in scope.

Fix

Recurse into flattened (and optional-flattened) dataclass fields when collecting field serializers / skip_if predicates, so they are available in the enclosing scope. Handles nested flattens too.

Tests

Added test_flatten_custom_field_serializer (basic + nested flatten; fails without the patch). Full suite green (5169 passed). Deserialization was already unaffected (a flattened child is deserialized via its own scoped function).

Thanks @AustinScola for the report and minimal reproducer.