feat: add integrity verification guards

[NightVibes33]

/claim #4

Summary

• replace the invalid native integrity exit path with named guarded exit codes
• add a C++ --print-integrity-crc32 helper plus normalized expected-CRC comparison for build-time integrity checks
• add anti_cheat.py with optional JAVELIN_EXPECTED_SHA256 script integrity verification
• document how to compute and set expected values for both implementations
• add focused Python regression coverage for missing, matching, malformed, mismatched, and tampered hashes

Verification

• python3 -m unittest discover -s tests -> 7 tests passed
• python3 -m py_compile anti_cheat.py tests/test_anti_cheat.py
• python3 anti_cheat.py -> [Javelin AntiCheat] All clear. Continue.
• git diff --check