Pressure for a stronger yuan is mounting. Last week Treasury secretary John W. Snow named a special envoy to engage China on exchange-rate issues, signaling a deepening commitment across the Bush administration to draw a harder line with Beijing. The administration began raising pressure on Beijing amid sharp criticism in Congress that Chinese companies get an unfair advantage in the market from Beijing’s currency-management practices, The result is well known, the trade deficit with China has been soaring to $155 billion last year.