feat: SHA-256 self-integrity verification (Python + C++)

[writsop]

Summary

Adds SHA-256 self-integrity verification to Javelin Anti-Cheat, complementing the existing CRC32 check. Includes a standalone Python implementation with full test suite.

Closes #4

Changes

AntiCheat.cpp (C++)

• Added zero-dependency SHA-256 implementation (RFC 6234 compliant)
• New checkSelfIntegritySHA256() function alongside existing CRC32
• Build-time constant JAVELIN_EXPECTED_SHA256 for baking hash into binary
• --compute-hash CLI flag to output both CRC32 and SHA-256 of the binary
• No new external dependencies

integrity_check.py (Python)

• Standalone SHA-256 integrity verification script
• Uses JAVELIN_EXPECTED_SHA256 env var for expected hash
• --compute-hash utility mode
• Graceful skip when env var not set (dev-friendly)
• Programmatic API: verify_integrity() and compute_file_sha256()

tests/test_integrity.py

• 11 unit tests, all passing
• Covers: known content hashing, empty files, large files (1MB), missing files, hash matching, mismatch detection, case insensitivity, whitespace handling, tamper detection

README.md

• Complete rewrite with feature matrix, quick start guides for both C++ and Python, architecture explanation, security notes

Testing

Ran 11 tests in 0.011s
OK

All tests pass on Python 3.10+. C++ compiles with MSVC and MinGW (Windows-only due to WinAPI dependencies).