PARIS — Into the valley of dreck dove the six hundred. Okay, not quite that many – there were fifty-five women who plunged into the Seine on Wednesday from the Pont Alexandre III. The call had come to their cardboard bunks at four o’clock in the morning: the scientists had counted the Escherichia coli yet again – une, deux, trois, quatre milliarde — and they had decreed in their white-coated wisdom that the hallowed French river, mortally poisonous just 24 hours earlier, suddenly was safe to swim and swallow.