Add Python SHA-256 integrity guard

[sureshchouksey8]

Summary

• Add javelin_integrity.py with optional SHA-256 verification controlled by JAVELIN_EXPECTED_SHA256.
• Fail closed with a guarded exit when the expected digest does not match the target script.
• Document how to generate and set expected hashes, plus the existing C++ CRC32 build-time constant flow.
• Add stdlib unittest coverage for matching hashes, mismatches, and guarded exit behavior.

Verification

• python3 -m unittest -q test_javelin_integrity.py
• python3 -m py_compile javelin_integrity.py test_javelin_integrity.py
• git diff --check

/claim #4