Add TODO for UNSIGNED cast emulation

MySQL wraps negative values on UNSIGNED casts using modular
arithmetic (2^64), but SQLite has no unsigned integer type.
Mark this as a known limitation to be addressed.

Author: JanJakes

packages/mysql-on-sqlite/src/sqlite/class-wp-pdo-mysql-on-sqlite.php

@@ -3790,6 +3790,8 @@ private function translate( $node ): ?string {
 						return 'TEXT';
 					case WP_MySQL_Lexer::SIGNED_SYMBOL:
 					case WP_MySQL_Lexer::UNSIGNED_SYMBOL:
+						// @TODO: Emulate UNSIGNED semantics. MySQL wraps negative
+						//        values, but SQLite has no unsigned integer type.
 						return 'INTEGER';
 					case WP_MySQL_Lexer::DECIMAL_SYMBOL:
 					case WP_MySQL_Lexer::FLOAT_SYMBOL:
@@ -4234,6 +4236,7 @@ private function translate_simple_expr_body( WP_Parser_Node $node ): string {
 
 			if ( null !== $cast_type ) {
 				// CONVERT(expr, type): Translate to cast expression.
+				// TODO: Emulate UNSIGNED cast. SQLite has no unsigned integer type.
 				return sprintf( 'CAST(%s AS %s)', $expr, $this->translate( $cast_type ) );
 			} else {
 				// CONVERT(expr USING charset): Keep "expr" as is (no SQLite support).

packages/mysql-on-sqlite/tests/WP_SQLite_Driver_Tests.php

@@ -9971,7 +9971,8 @@ public function testCastExpression(): void {
 					'expr_5'  => 'abc',
 					'expr_6'  => 'abc',                 // 'ab' In MySQL
 					'expr_7'  => '-10',
-					'expr_8'  => '-10',                 // 18446744073709551606 in MySQL
+					// @TODO: Emulate UNSIGNED cast. MySQL returns 18446744073709551606 (2^64 - 10).
+					'expr_8'  => '-10',
 					'expr_9'  => '2025-10-05 14:05:28', // 2025-10-05 in MySQL
 					'expr_10' => '2025-10-05 14:05:28', // 14:05:28 in MySQL
 					'expr_11' => '2025-10-05 14:05:28',
@@ -10005,6 +10006,7 @@ public function testConvertExpression(): void {
 					'expr_1' => 'abc',
 					'expr_2' => 'abc',
 					'expr_3' => '-10',
+					// @TODO: Emulate UNSIGNED cast. MySQL returns 18446744073709551606 (2^64 - 10).
 					'expr_4' => '-10',
 					'expr_5' => '123.456',
 					'expr_6' => '2025-10-05',