feat: arbitrary GF(2^k) support

[lonerapier]

Extend beyond GF(256) to arbitrary binary extension fields (GF(2⁴), GF(2⁸), GF(2¹⁶), etc.) for flexible security/performance trade-offs.

Approach

• Generalize field implementation to support configurable polynomial degrees.
• Use const generics for GF types.
• Maintain compile-time irreducible polynomial validation.

Tasks:

• Refactor current GF256 module into generic gf2k crate or use an audited field crate.
• Define list of supported irreducible polynomials.
• Add tests and benches for multiple k values.