feat: persist workflow signals and arbitrate poll completion

internal/storage/mysql/workflow_apply.go

@@ -255,6 +255,11 @@ func activateWorkflowStep(ctx context.Context, tx *sql.Tx, g *workflowGraph, row
 			if err := saveWorkflowClock(ctx, tx, g, row, "TIMER", row.view.Revision+1, until); err != nil {
 				return err
 			}
+		} else if c.Wait.Kind == "SIGNAL" {
+			// 先扫描早到信号；未匹配时运行器按原等待截止继续持久化。
+			if err := saveWorkflowClock(ctx, tx, g, row, "SIGNAL", row.view.Revision+1, now); err != nil {
+				return err
+			}
 		}
 	}
 	if c.Task != nil {
@@ -274,7 +279,11 @@ func activateWorkflowStep(ctx context.Context, tx *sql.Tx, g *workflowGraph, row
 		}
 	}
 	if c.Kind == "POLL" {
-		return saveWorkflowClock(ctx, tx, g, row, "POLL", row.view.Revision+1, deadline)
+		due := deadline
+		if c.Wait != nil {
+			due = now
+		}
+		return saveWorkflowClock(ctx, tx, g, row, "POLL", row.view.Revision+1, due)
 	}
 	return nil
 }