Add P256VERIFY precompile (EIP-7951)

[mw2000]

Summary

Add a precompile for secp256r1 (P-256) signature verification. Activates with Fusaka.

Spec

• EIP-7951: Precompile for secp256r1 Curve Support — https://eips.ethereum.org/EIPS/eip-7951

Scope

• Add Fusaka hardfork + EIP-7951 flag to EEVM.HardforkConfig.
• Implement EEVM.Precompiles.P256Verify at the assigned address (0x100).
• Wire it into the precompile dispatcher behind the flag.
• Likely needs a NIF or pure-Elixir secp256r1 implementation.

Definition of Done

• Returns a 32-byte 0x...01 for valid signatures and empty output for invalid ones.
• Fusaka P256VERIFY StateTest fixtures pass.

Parent

• EVM Completeness: Missing opcodes, precompiles, and gas rules #30 — EVM Completeness tracking issue