fix(hook-bus): write_to_flat 改用 TMDO_DB::upsert（B4）

原本 SELECT id → UPDATE/INSERT 三步，並發寫入同一 entity 會產生
duplicate row 或互相覆蓋。改為單一 upsert（A v3.1.7 同款）。

unit 379 / integration 398 GREEN

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TbG1keQQ7XBa7qMQY16KCY

includes/engine/class-tmdo-hook-bus.php

@@ -321,35 +321,18 @@ final class TMDO_Hook_Bus {
 		self::$internal_ops[ $lock_key ] = true;
 
 		try {
-			// 檢查列是否存在
-			$exists = $wpdb->get_var(
-				$wpdb->prepare(
-					"SELECT id FROM `{$table}` WHERE `{$id_col}` = %d",
-					$entity_id
-				)
+			// 單一 upsert，避免 SELECT→UPDATE/INSERT 的 read-modify-write race。
+			$result = TMDO_DB::upsert(
+				$table,
+				array(
+					$id_col => $entity_id,
+					$col    => $db_value,
+				),
+				array( $col ),
+				$id_col,
+				array( '%d', $format )
 			);
 
-			if ( $exists ) {
-				// UPDATE
-				$result = $wpdb->update(
-					$table,
-					array( $col => $db_value ),
-					array( $id_col => $entity_id ),
-					array( $format ),
-					array( '%d' )
-				);
-			} else {
-				// INSERT
-				$result = $wpdb->insert(
-					$table,
-					array(
-						$id_col => $entity_id,
-						$col    => $db_value,
-					),
-					array( '%d', $format )
-				);
-			}
-
 			// 清除快取
 			TMDO_Cache_Orchestrator::invalidate( $type, $entity_id, $group );