feat: support unevaluatedProperties in Draft 2019-09

[tomatotomata]

Summary

This adds a focused proof of concept for unevaluatedProperties in Draft 2019-09, following the scope discussed in #907.

The implementation derives evaluated property names from properties, patternProperties, and direct allOf branches. That covers the motivating composition while keeping the first patch small. When unevaluatedProperties is a schema, each remaining property is validated against it. When it is false, the validator reports the property with a dedicated constraint error.

The validator does not yet carry annotations across every applicator, so this intentionally does not claim complete Draft 2019-09 semantics for anyOf, oneOf, conditionals, or referenced schemas. I would treat this as a proof of concept for the project to shape further.

Tests

• php -d extension=mbstring -d extension=openssl vendor/bin/phpunit --filter UnevaluatedPropertiesTest --testdox passes locally (2 assertions-backed cases, 2 expected skips for the assoc/type-cast variants).
• PHPStan passes for the new constraint.
• git diff --check passes.
• The pre-existing full-suite run reaches the suite but has an unrelated Windows path-separator failure; the original CI run also exposed the missing Draft 2019 strict-mode flag in this new test, which is fixed in the follow-up commit.

This patch was prepared with AI assistance and then reviewed and tested in this checkout. The repository's contribution notice asks contributors to have authored 100% of the content, so please let me know if the project needs a different treatment of that requirement before considering the patch.

Updates #907