Sheela Foam, the company behind the familiar Sleepwell mattress, is in the thick of a challenging transition. The news that CEO Nilesh Sevabrata Mazumdar has stepped down just as the company faces its biggest integration test could understandably make investors anxious.The company’s shares, which peaked around the time of the Kurlon acquisition, have since tumbled nearly 50% and now trade below the IPO issue price, a sharp reversal for a business once celebrated for steady, profitable growth.