fix(processing): 收紧消息生命周期与投递约束

src/main/kotlin/com/gzzn/omms/msgexchange/domain/ProcState.kt

@@ -72,5 +72,7 @@ data class ProcState(
     val backfillNextAt: Instant? = null,
     val backfillAttempts: Int = 0,
     val backfillError: String? = null,
+    /** 首次被主泵取得的时刻；重试不刷新，用作 HOL deadline 的稳定起点。 */
+    val processingStartedAt: Instant? = null,
     val updatedAt: Instant = Instant.now(),
 )