Add robust integrity verification guards

[kitwongpixel]

/claim #4

Summary

• Adds a C++ executable CRC32 integrity guard with a canonical build-time marker so embedding the expected CRC does not change the verified bytes.
• Adds --print-integrity-crc32 for C++ setup and exits with 0xC0 on integrity mismatch.
• Adds a Python SHA-256 script integrity guard driven by JAVELIN_EXPECTED_SHA256, plus --print-integrity-sha256.
• Documents expected-value setup, including the Linux -Wl,--build-id=none note needed for reproducible C++ CRC builds.
• Adds focused regression tests proving C++ configured-match accepts, C++ mismatch rejects, and Python match/mismatch behavior works.

Demo

Short demo video: https://github.com/kitwongpixel/py-workedtask/blob/codex/issue-4-robust-integrity/docs/javelin-integrity-demo.mp4

Transcript: docs/demo-transcript.txt

Validation

• python3 -m unittest discover -s tests -v -> 7 tests OK
• python3 -m py_compile anti_cheat.py tests/test_integrity.py
• git diff --check