perf(vanillastark): switch to ark-ff Montgomery multiplication, add criterion benchmarks

Replace manual Russian-peasant field arithmetic with ark-ff's Montgomery
backend. Field mul: 287ns → 2.3ns (126x), STARK prove: 658ms → 22ms (30x),
STARK verify: 31ms → 1ms (31x). Add criterion benchmark suite covering
field ops, polynomial ops, Rescue-Prime, FRI, and full STARK prove/verify.

Author: AbdelStark