Add a hook for native MySQL lexer and parser implementations

[adamziel]

Summary

Add a small entry point in load.php so an optional native (e.g. Rust) MySQL lexer/parser extension can plug itself in as WP_MySQL_Lexer / WP_MySQL_Parser. The extension pre-declares WP_MySQL_Native_Lexer / WP_MySQL_Native_Parser before this file loads; otherwise the existing pure-PHP classes ship as the implementation, unchanged.

The existing PHP files keep their original names, locations, and class names — no rename, no polyfill scaffolding. Native shims live in a separate mysql/native/ directory and are only required when the native classes are present.

Testing

• php -l on changed files
• Smoke: WP_MySQL_Lexer and WP_MySQL_Parser instantiate and parse SELECT 1 end-to-end with no native extension loaded
• phpcs clean

[JanJakes]

@adamziel The changes here are clear. I just have two API/naming observations:

1. The "polyfill" naming feels a bit misleading. In many use cases, and especially in the plugin context, the PHP classes are the source of truth and the only possible implementation.

2. Related to 1, is the WP_MySQL_Parser extends WP_MySQL_Polyfill_Parser part necessary? Would it be possible to just require the existing classes conditionally in https://github.com/WordPress/sqlite-database-integration/blob/trunk/packages/mysql-on-sqlite/src/load.php without even moving and renaming them:

if ( class_exists( 'WP_MySQL_Native_Parser', false ) ) {
    class WP_MySQL_Parser extends WP_MySQL_Native_Parser {}
} else {
    require_once __DIR__ . '/mysql/class-wp-mysql-parser.php';
}

Or, if both have the same class name, just:

if ( ! class_exists( 'WP_MySQL_Parser', false ) ) {
    require_once __DIR__ . '/mysql/class-wp-mysql-parser.php';
}

That way, the existing code would remain largely as-is, and we'd just add some native extension entrypoint.

Btw. If we need to stick with renames, I don't know why, but GitHub diff doesn't see them as renames. Perhaps that's fixable for better history and blames?

[adamziel]

@JanJakes I like this option:

if ( class_exists( 'WP_MySQL_Native_Parser', false ) ) {
    class WP_MySQL_Parser extends WP_MySQL_Native_Parser {}
} else {
    require_once __DIR__ . '/mysql/class-wp-mysql-parser.php';
}

It's implemented now :) I'll fix PHPCS and we may be good to go here

[JanJakes]

@adamziel Amazing, thanks!