native: support aliased Meta Query self joins

[chubes4]

Summary

• accept arbitrary AND/OR ON predicates and cross-alias equality/range comparisons
• evaluate ON before LEFT JOIN null extension, then apply WHERE to joined rows
• materialize aliased parenthesized SELECT, UNION, and UNION ALL sources in FROM and JOIN
• preserve SQL UNION DISTINCT sequencing after UNION ALL and bound indexed join fan-out by actual candidate pairs
• add raw and WordPress WP_Query multi-alias Meta Query coverage

Verification

• php tests/smoke-native-join-query.php
• php tests/smoke-native-subquery-union.php
• php tests/smoke-native-option-query.php
• PHP lint and git diff --check
• isolated Lab checkout: complete tests/smoke-*.php sweep
• isolated Lab WordPress Codebox: MDI_WP_CODEBOX_BIN="$HOME/.local/bin/wp-codebox" php tests/run-native-mysql-coverage.php
  • 38/38 statements and 26/26 operations supported

Notes

Generic non-equality joins use a 100,000 candidate-pair cost limit; equality joins retain indexed lookup behavior and now count actual fan-out. The sibling #372 scalar review is independent.

AI assistance

Implemented and tested with OpenAI GPT-5.6 Terra (openai/gpt-5.6-terra) via OpenCode. I extended the typed parser, query contracts, and native executor; independently audited SQL result semantics; added behavioral regressions; and ran local, isolated Lab smoke, and WordPress Codebox validation.