feat: add SHA-256 integrity verification for C++ and Python (#4)

[promisingcoder]

Description

Implements optional SHA-256 integrity verification for both C++ and Python implementations, as requested in #4.

/claim #4

Changes

C++ (AntiCheat.cpp)

• Added SHA-256 computation using Windows-native bcrypt.h (BCryptHashData) — no external dependencies
• Added JAVELIN_EXPECTED_SHA256 build-time constant (defaults to "" = disabled)
• Added checkSelfIntegritySHA256() function that reads the running executable, computes SHA-256, and compares to expected hex
• Called in main() after existing CRC32 check — exits with 0x54A on mismatch
• Existing CRC32 check left intact

Python (anti_cheat.py) — new file

• compute_sha256(filepath) — returns hex digest using hashlib
• verify_integrity() — checks SHA-256 against JAVELIN_EXPECTED_SHA256 env var
• Skips verification when env var is not set (optional verification)
• Exits with code 1 on mismatch
• Also includes basic anti-cheat functions (detect_suspicious_processes, is_debugger_present)

Documentation (README.md)

• Full setup instructions for both C++ and Python SHA-256 verification
• How to compute expected hash values (PowerShell, bash, Python methods)
• Exit code reference table
• API reference

Acceptance Criteria

• Non-matching hash results in guarded exit (both C++ and Python)
• Document how to set the expected values

fixes #4