By Karin Strohecker LONDON (Reuters) – With much of Moscow’s $640 billion reserves under lock and key in the West and sanctions crippling cross-border capital flows, investors fear Russia may be heading for its first ever default on sovereign hard currency debt. On Wednesday, foreign investors were effectively stuck with their holdings of rouble-denominated bonds — known as OFZs — after the central bank temporarily halted coupon payments and settlement system Euroclear stopped accepting Russian assets. A rouble debt default has precedent — Moscow reneged on OFZs during its 1998 financial cr…