perf(2meet-data-optimizer): flush the cold cache group without a server-side scan

- TMDO_Cache_Layer::flush_group() bumps the post type's cold cache version
  (option wpdo_cold_cache_version) instead of wp_cache_flush_group(), which
  Redis Object Cache runs as one server-side scan that blocks Redis on a large
  group; entries under older keys expire by their TTL
- TMDO_Zone_Cold::cache_key() builds every cold blob key; version 0 keeps the
  current keys, so upgrading invalidates nothing
- Verify: CacheLayerTest (new keys, no scan, other post types untouched),
  unit 702, integration 583, phpcs, stan
- PR: PR-110

tests/unit/CacheLayerTest.php

@@ -197,6 +197,25 @@ class CacheLayerTest extends TestCase {
 		$this->assertNotSame( [ 'stale' => true ], $GLOBALS['_wp_cache'][ $group ][ $cache_key ] ?? null );
 	}
 
+	/**
+	 * Flushing bumps the post type's cache version instead of asking the
+	 * object cache to scan the group (a server-side script on Redis that
+	 * blocks it on a large group); older entries expire by their TTL.
+	 */
+	public function test_flush_group_moves_to_new_keys_without_a_group_flush(): void {
+		$group                                   = 'wpdo_cold_hp_listing';
+		$old_key                                 = WPDO_Zone_Cold::cache_key( 1, 'hp_listing' );
+		$GLOBALS['_wp_cache'][ $group ][ $old_key ] = array( 'stale' => true );
+
+		WPDO_Cache_Layer::flush_group( 'hp_listing' );
+
+		$new_key = WPDO_Zone_Cold::cache_key( 1, 'hp_listing' );
+		$this->assertNotSame( $old_key, $new_key );
+		$this->assertFalse( wp_cache_get( $new_key, $group ) );
+		$this->assertSame( 'cold_2', WPDO_Zone_Cold::cache_key( 2, 'other_type' ), 'other post types keep their keys' );
+		$this->assertArrayHasKey( $old_key, $GLOBALS['_wp_cache'][ $group ], 'nothing scanned or deleted' );
+	}
+
 	// ── get_stats() ──────────────────────────────────────────────────────────
 
 	public function test_get_stats_includes_required_keys(): void {