feat(2meet-data-optimizer): support composite indexes on entity groups

- register_group() accepts $args['indexes'] (name => ordered keys); indexes on
  unknown keys are dropped. Kept outside the field definition, so schema
  hashes are unchanged.
- Schema_Manager::ensure_group_indexes() adds missing cidx_* indexes to the
  existing flat table in place; the applied definition is remembered in
  tmdo_entity_indexes so later requests skip SHOW INDEX.
- Bench (1M listings, scenario 2): Entity p50 337 ms -> 278 ms with
  (hp_verified, hp_price), on par with Hot (269 ms).
- Verify: core unit 656 / integration 459 green, phpcs clean
- PR: PR-027

includes/engine/class-tmdo-schema-manager.php

@@ -20,6 +20,9 @@ defined( 'ABSPATH' ) || exit;
 
 final class TMDO_Schema_Manager {
 
+	/** Option: "entity/group" → md5 of the composite index definition already applied. */
+	public const INDEXES_OPTION = 'tmdo_entity_indexes';
+
 	/** Request-scoped cache for table_exists() — prevents repeated SHOW TABLES per request. */
 	private static array $table_exists_cache = array();
 
@@ -60,6 +63,7 @@ final class TMDO_Schema_Manager {
 				$schema['group'],
 				$schema['fields']
 			);
+			self::ensure_group_indexes( $schema['type'], $schema['group'] );
 		}
 
 		TMDO_Entity_Registry::clear_pending_schemas();
@@ -164,6 +168,50 @@ final class TMDO_Schema_Manager {
 		return true;
 	}
 
+	/**
+	 * Add the group's declared composite indexes that its table lacks.
+	 *
+	 * Indexes are not part of the schema hash, so an existing table (which
+	 * may already hold aeav_only data) gets them in place — no rebuild. The
+	 * applied definition is remembered per group (one autoloaded option), so
+	 * requests only run SHOW INDEX / ALTER when the declaration changes.
+	 *
+	 * @param string $entity_type Entity type.
+	 * @param string $group_name  Group name.
+	 * @return void
+	 */
+	public static function ensure_group_indexes( string $entity_type, string $group_name ): void {
+		global $wpdb;
+		$declared = TMDO_Entity_Registry::get_group_indexes( $entity_type, $group_name );
+		if ( empty( $declared ) ) {
+			return;
+		}
+		$id        = "{$entity_type}/{$group_name}";
+		$signature = md5( (string) wp_json_encode( $declared ) );
+		$applied   = get_option( self::INDEXES_OPTION, array() );
+		$applied   = is_array( $applied ) ? $applied : array();
+		if ( ( $applied[ $id ] ?? '' ) === $signature ) {
+			return;
+		}
+		$table = self::get_table_name( $entity_type, $group_name );
+		if ( ! self::table_exists( $table ) ) {
+			return;
+		}
+
+		$existing = array_unique( array_column( (array) $wpdb->get_results( "SHOW INDEX FROM `{$table}`", ARRAY_A ), 'Key_name' ) );
+		foreach ( $declared as $name => $keys ) {
+			$index = 'cidx_' . self::sanitize_column_name( $name );
+			if ( in_array( $index, $existing, true ) ) {
+				continue;
+			}
+			$columns = implode( ', ', array_map( static fn( $key ) => '`' . self::sanitize_column_name( $key ) . '`', $keys ) );
+			$wpdb->query( "ALTER TABLE `{$table}` ADD INDEX `{$index}` ({$columns})" );
+		}
+
+		$applied[ $id ] = $signature;
+		update_option( self::INDEXES_OPTION, $applied, true );
+	}
+
 	/**
 	 * 計算 Schema Hash（用於偵測欄位變動）
 	 */