President Clinton, at the central photo-op of his six-country Mideast tour, said today's signing of the treaty between Israel and Jordan made peace in the region "unstoppable." After presiding over the affair with Jordan's King Hussein and Israeli Prime Minister Yitzhak Rabin -- the podium, symbolically, was erected on a former mine field on the two countries' border -- Clinton made a peace offering to uneasy Jordanian Muslims. "We respect Islam," he announced, to loud applause from at least one side of the river Jordan. That appeared to set up his next line, a condemnation of Islamic terrorists "who cloak themselves in the rhetoric of religion and nationalism . . . You cannot succeed, you will not succeed, you must not succeed, for you are the past, not the future." (Some Jordanians, in spite of their popular king, have praised Hamas, the group responsible for recent attacks on Israelis.) The treaty itself, which formally ended a half-century of emnity between Israel and Jordan, defines borders, water-sharing and trade between the two countries.