WASHINGTON/BEIJING, July 7 (Reuters) - As U.S. Treasury Secretary Janet Yellen tries to steady an increasingly tense relationship with China, the world's two largest economies remain tightly linked, but are showing some signs of future weakness. Yellen starts meetings on Friday with senior Chinese officials pledging to seek "healthy competition" with China as confrontation between the economic powers over U.S. technology export controls and planned outbound investment restrictions dominate headlines. Despite talk of U.S.-China economic de-coupling, recent data show a trade relationship that is fundamentally solid, and rebounded in 2022 from five years of turmoil wrought by a trade war...