Treat deserializer compatibility as serializer output

[ostrowr]

Summary

• interpret Role::Deserializer against values the old serializer could emit, instead of every value the old schema would accept through undeclared additional properties
• thread that emitted-value assumption through object subset checks
• add a focused regression test for adding an optional property under default-open object semantics

Why

The current checker treats open object schemas as if old serializers might emit arbitrary undeclared keys. That makes an added optional property look deserializer-incompatible even when the serializer contract is "only declared fields are emitted," which is the behavior described in the README.

Validation

• cargo test check_compat_treats_optional_added_property_as_deserializer_compatible --quiet
• cargo test --test backcompat complex_optional_prop_added --quiet
• cargo fmt --all --check

Follow-up

cargo test --quiet still fails in tests/backcompat.rs for additionalprops_reduced and pattern_properties_fall_back_to_additional_properties. Those fixtures and the fuzz confirmation harness still encode the old deserializer semantics for open object schemas, so they need a follow-up decision/update before this should leave draft.