TAREK AMARA and TOM PERRY PRESIDENT Kais Saied looks on course to tighten his grip on Tunisia through a constitutional referendum in July, but it could prove to be a poisoned chalice as the economy sinks deeper into crisis and opposition to his rule widens. Nearly a year since Saied began amassing power, the July 25 referendum is widely expected to boost his authority, in what critics see as a march to one-man rule that has unpicked the democratic gains of Tunisia's 2011 "Arab Spring" uprising. For Saied, overhauling the 2014 constitution is a corrective to political dysfunction which had…