PARIS, Sept 21 (Reuters) - Bernard Arnault, the boss of luxury goods group LVMH and France's richest man, has attacked a proposed 2% tax on billionaires as an assault on France's economy and denounced the plan's architect as a far-left ideologue. The tax, which would target wealth above 100 million euros ($117 million), has gained political traction in France, where Prime Minister Sébastien Lecornu faces pressure from the Socialist Party to include it in the 2026 budget or face a confidence vote that could topple his government. He accused the plan's architect, economist Gabriel Zucman of being "first and foremost...