Add optional native parser routing

[adamziel]

Summary

• add fallback WP_MySQL_Native_* classes that extend the PHP polyfills
• route public MySQL lexer/parser classes through native classes when the extension preloads them
• add the minimal PHP bridge that exports grammar data for native parser setup

Testing

• php -l on changed PHP files
• parser smoke: lex and parse SELECT 1 through WP_MySQL_Lexer and WP_MySQL_Parser

This PR is intentionally stacked after the polyfill extraction PR so the native routing diff is small and reviewable.