ff-macros: reject zero unroll factor in unroll_for_loops

[sashass1315]

Previously, the unroll_for_loops attribute accepted 0 as a valid integer because syn parses it as Lit::Int and we converted it to usize without additional checks. While negatives were already impossible (they parse as a unary minus over an int and are rejected), zero slipped through. This led to generated code in unroll.rs performing division and modulo by the unroll factor, causing a guaranteed runtime panic when unroll_by == 0.
This change adds an explicit guard in ff-macros/src/lib.rs to reject unroll_by == 0 with the same ARG_MSG (“must be a positive integer”), enforcing the documented constraint and converting a runtime panic into a compile-time error. No API changes; existing valid usages (e.g., 6, 12) are unaffected.