Add experimental DuckDB backend support

[adamziel]

What it does

Adds an experimental optional duckdb backend alongside the existing SQLite path.

The default SQLite runtime stays zero-dependency. DuckDB is only selected when DB_ENGINE=duckdb and the DuckDB PHP client autoloader is supplied through DUCKDB_PHP_AUTOLOAD.

Rationale

DuckDB gives the project a second SQL execution backend for local development, compatibility experiments, and analytical-style database work. It has different runtime requirements from SQLite, so the implementation is explicitly gated behind runtime checks for PHP/FFI/client availability.

This PR keeps unsupported MySQL behavior explicit: unsupported statements fail through WP_DuckDB_Driver_Exception instead of silently pretending to work.

Implementation

Adds the DuckDB driver under packages/mysql-on-sqlite/src/duckdb:

• connection, prepared statement, result statement, runtime gate, and driver exception wrappers
• MySQL-to-DuckDB translation for common WordPress DDL, DML, metadata, expression, and wpdb query shapes
• metadata emulation for MySQL-shaped information_schema, SHOW, DESCRIBE, indexes, constraints, and table status queries
• WordPress drop-in support via DB_ENGINE=duckdb
• DuckDB-specific PHPUnit, WordPress PHPUnit, WordPress E2E, smoke, and runtime setup paths
• CI fast-fail guards and 30-minute PR workflow caps for long DuckDB/WASM paths
• strict full WordPress PHPUnit coverage for DuckDB, with expected-result allowlists disabled
• fast paths for high-frequency WordPress PHPUnit transaction/autocommit boilerplate

The DuckDB path remains experimental. The SQLite path remains the default runtime.

Testing instructions

Default suite:

cd packages/mysql-on-sqlite
composer run test

Coding standards:

composer run check-cs

Live DuckDB suite, with a DuckDB PHP autoloader installed separately:

cd packages/mysql-on-sqlite
WP_DUCKDB_TESTS=1 \
WP_DUCKDB_AUTOLOAD=/path/to/duckdb/vendor/autoload.php \
DUCKDB_PHP_AUTOLOAD=/path/to/duckdb/vendor/autoload.php \
php -d ffi.enable=1 ./vendor/bin/phpunit -c ./phpunit.xml.dist --group duckdb

Current status

Current PR head: 21d5965.

Verified locally:

• composer check-cs -- --no-cache --report-full --report-checkstyle=./phpcs-report.xml
• WP_DUCKDB_TESTS=1 ... php -d ffi.enable=1 packages/mysql-on-sqlite/vendor/bin/phpunit -c packages/mysql-on-sqlite/phpunit.xml.dist --group duckdb
• focused DuckDB transaction/found-rows fast-path tests

GitHub status at this update:

• CS: passing on 21d5965
• MySQL Proxy Tests: passing on 21d5965
• Verify release metadata: passing on 21d5965
• full WordPress/DuckDB CI: queued or running; not yet claimed green