Implement SHA-256 precompile (0x02)

Summary

Implement the SHA-256 precompile at address 0x02.

Prerequisites

• #34 — Add precompile routing in CALL path

Specification

• Address: 0x02
• Input: arbitrary bytes
• Output: 32-byte SHA-256 hash
• Gas: 60 base + 12 per word (32-byte chunks, rounded up)

Implementation Guide

1. Create lib/eevm/precompiles/sha256.ex:
   • Gas: 60 + 12 * ceil(byte_size(input) / 32)
   • Hash: :crypto.hash(:sha256, input)
2. Register in Precompiles.execute/3 for address 0x02.
3. Tests — empty input hash, known test vectors, gas calculation, out of gas.

Acceptance Criteria

• [ ] Returns correct SHA-256 hash
• [ ] Gas calculated correctly
• [ ] Tests pass

Reference

• Yellow Paper §E.2