native: expand scalar projection functions

[chubes4]

Summary

• Preserve scalar-containing WHERE clauses as generic OR-of-AND boolean groups, including WordPress Date Query-style OR with a normal conjunct.
• Force scalar residual filters to run before LIMIT; apply scalar HAVING after aggregate grouping.
• Fail closed instead of silently bypassing scalar clauses on JOINs or scalar/subquery mixtures.

Verification

• php -l inc/native/class-wp-markdown-native-query-{ast,contracts,parser,executor}.php
• php tests/smoke-native-scalar-projection.php
• php tests/smoke-native-scalar-clauses.php
• Lab: isolated native scalar smoke passed.
• Lab: disposable MariaDB 11.4 matched Date Query OR (1,3), calendar buckets (2024-01:2,2024-02:1), and geo order (3,2,1).
• Lab: WordPress Codebox coverage completed and recognized all three scalar statement surfaces.

Limits

• The Codebox coverage report is not a complete pass (38/39 statements, 16/24 operations), due to unrelated existing unsupported shapes.
• SQLite differential smoke is blocked before comparison by a missing generated result_envelope artifact in this checkout.
• Scalar work is residual after a full safe read; scalar JOIN/subquery mixtures fail closed. Grouped-result ORDER BY/LIMIT is still unsupported.

AI Assistance Disclosure

Implemented and tested with OpenAI GPT-5.6 Terra (openai/gpt-5.6-terra) using OpenCode. I reviewed and repaired parser/plan/executor semantics, added regression coverage, and ran the listed native, MariaDB, and WordPress Codebox workflows.