Add Python SHA-256 integrity verification

[fxpolice1-sketch]

Implements #4.

Implements optional Python script integrity verification and documents both C++ and Python expected-value setup.

What changed:

• Added AntiCheat.py with SHA-256 verification of the script file.
• The Python guard compares against JAVELIN_EXPECTED_SHA256 when the env var is set.
• Non-matching hash exits through a guarded failure path with a non-zero status.
• Updated README.md with C++ CRC32 build-time setup and Python SHA-256 env var setup.

Validation:

python -B -m py_compile AntiCheat.py

# matching hash
JAVELIN_EXPECTED_SHA256=<computed-hash> python AntiCheat.py
exit: 0

# mismatching hash
JAVELIN_EXPECTED_SHA256=deadbeef python AntiCheat.py
exit: 81