Add optional integrity verification

[0xdungki]

Summary

• Adds a Python anti-cheat guard with optional SHA-256 script integrity verification via JAVELIN_EXPECTED_SHA256.
• Documents how to generate/set expected Python SHA-256 hashes and how to configure the C++ build-time CRC32 value.
• Fixes the invalid C++ integrity failure return literal and keeps the existing CRC32 self-check flow guarded by JAVELIN_EXPECTED_CRC32.
• Adds Python unit tests for skipped, matching, and mismatched integrity checks.

Verification

• python3 -m unittest -v

Fixes #4