feat: add strict SHA-256 integrity verification for issue #4

[rbxict]

Summary

• add strict SHA-256 integrity verification for both implementations in issue #4
• verify the running executable in AntiCheat.cpp against a build-time JAVELIN_EXPECTED_SHA256 constant while keeping the existing guarded-exit behaviour
• add a Python reference implementation in anti_cheat.py, document the expected-hash setup, and include regression coverage for match, mismatch, malformed hash, missing env var, and tamper detection paths

Validation

• python -m py_compile anti_cheat.py
• python -m unittest discover -s . -p "test_*.py" -v

Notes

• the C++ implementation is source-complete, but I did not compile it in this environment because the MSVC cl toolchain is not installed here

Payout Details

• EVM: 0xe744f6791a685b0A0cC316ED44375B69361c837F
• SOL: 8BsByR6rPqxDPku6dYtdoiSk6bdgE9YETbLQF2RGSw1C

/claim #4

Closes #4
PoA: poa_e144a29c3deb803f

[rbxict]

I force-pushed a metadata-only follow-up so the branch carries a real CLI-generated PoA in the commit body instead of leaving the PR without one. I also cleaned up the PR description and locally re-ran the Python verification: python -m py_compile anti_cheat.py and python -m unittest discover -s . -p "test_*.py" -v both pass on this checkout.