Indian Prime Minister Narendra Modi's Bharatiya Janata Party (BJP) won a decisive election victory in opposition-held West Bengal , a key eastern state of more than 100 million people, marking a dramatic shift in a state long seen as resistant to the party's rise.   According to results announced Monday, the BJP secured 206 of the 294 assembly seats. The result allows the BJP to expand its political dominance beyond the Hindi-speaking heartland of northern and central India. West Bengal, a largely Bengali-speaking state, had been ruled by a regional party, the All India Trinamool Congress (TMC), since 2011. The party's leader Mamata Banerjee has been a fierce critic of Modi. But the latest election marked a steep downfall for the three-term chief minister, with Mamata losing her own seat. Experts say she was voted out largely on anti-incumbency sentiments over mounting criticism of corruption and lack of economic opportunities in the state.…