Tokyo stocks opened flat Monday, as buying amid improved sentiment after a sustained rally the previous week was offset by moves to lock in gains. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 0.04 point from Friday to 35,577.07. The broader Topix index was up 5.33 points, or 0.21 percent, at 2,499.56. On the top-tier Prime Market, gainers were led by marine transportation and securities house issues, while top decliners were air transportation and service issues. At 9 a.m., U.S. the dollar fetched 145.14-17 yen compared with 144.87-97 yen in New York and 145.14-1…