perf(2meet-data-optimizer): let flat-first sorts use a (column, id) index

- a group index may end with the entity ID column (post_id, ...)
- flat-first rewrites the posts.ID tie-breaks once, in the sort's direction:
  with a (column, post_id) index MySQL returns rows in order instead of
  sorting the whole match. dev36 (1.3M listings), price range sorted by price:
  850 ms filesort -> 2 ms. Ties are ordered by ID the same way as the column
  (natively ID DESC); pagination stays stable, result sets are unchanged
- Verify: EntityRegistryIndexesTest, PostQueryRouterFoundRowsTest (ASC
  tie-break, single tie-break), unit 704, integration 589, phpcs, stan;
  dev36 with the HivePress indexes (PR-119)
- PR: PR-118

includes/query/class-tmdo-post-query-router.php

@@ -797,10 +797,17 @@ class TMDO_Post_Query_Router {
 		if ( 1 === preg_match_all( '/\bJOIN\b/i', (string) $flat_first ) ) {
 			$flat_first = preg_replace( '/\s+GROUP BY\s+`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?=\s)/', ' ', (string) $flat_first, 1 );
 		}
-		// Tie-breaks on posts.ID (ours, and any a plugin appended) read the flat post_id.
+		// Tie-breaks on posts.ID (ours, and any a plugin appended) read the flat
+		// post_id, once, in the sort's direction: a (column, post_id) index then
+		// returns rows in order with no filesort. Ties are ordered by ID the same
+		// way as the column (natively ID DESC); pagination stays stable.
+		$tie        = "`{$alias}`.`post_id` " . ( 'ASC' === strtoupper( (string) $sort['order'] ) ? 'ASC' : 'DESC' );
 		$flat_first = preg_replace_callback(
 			'/\bORDER BY\b(?!.*\bORDER BY\b).*$/s',
-			static fn( $m ) => preg_replace( '/`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?!\w)/', "`{$alias}`.`post_id`", $m[0] ),
+			static function ( $m ) use ( $wpdb, $tie ) {
+				$order = (string) preg_replace( '/`?' . preg_quote( $wpdb->posts, '/' ) . '`?\.`?ID`?(?!\w)(\s+(ASC|DESC)\b)?/i', $tie, $m[0] );
+				return (string) preg_replace( '/(,\s*' . preg_quote( $tie, '/' ) . ')(\s*,\s*' . preg_quote( $tie, '/' ) . ')+/', '$1', $order );
+			},
 			(string) $flat_first
 		);
 		return (string) preg_replace( '/^\s*SELECT\s/', 'SELECT STRAIGHT_JOIN ', (string) $flat_first, 1 );