template mulAcc[T: uint64 | uint32](t, u, v: T; a, b: T): tuple[t, u, v: T]
Calculates: (t, u, v) <- (t, u, v) + a * b.
Used for Comba multiplication column accumulation.
template mulDoubleAdd2[T: uint64 | uint32](a, b, c, dHi, dLo: T): tuple[ r2, r1, r0: T]
Calculates: (r2, r1, r0) = 2 * a * b + c + (dHi, dLo).
Returns (r2, r1, r0) where r2 is the overflow carry (0 or 1).