For the first time in thirty years, Japan's long-term borrowing costs have climbed to a level that forces a reckoning — not only with inflation, but with the fragility of growth beneath it. The 10-year government bond yield reached 2.925 percent on Monday, carried upward by six consecutive sessions of selling that mirrored a global retreat from sovereign debt. At the heart of the uncertainty lies the Bank of Japan, whose path forward — how far, how fast, and where it stops — remains unresolved, even as the economy it must navigate delivered growth less than half of what markets had anticipated