Rename misleading variable in radix2 FFT

[maradini77]

PR Description

• Replace duplicity_of_initials with duplication_factor in poly/src/domain/radix2/fft.rs.
• Improves clarity: “duplicity” (deceit) → “duplication” (repeat count), matching actual behavior.

Rationale

• Reduces cognitive load and risk of misreading in FFT logic.
• Aligns naming with the variable’s role: initial value replication factor.