Add optional C++ and Python integrity verification

[VexCode24]

Fixes #4.

Summary:

• Adds a Python anti-cheat entry point that verifies its own SHA-256 when JAVELIN_EXPECTED_SHA256 is configured.
• Keeps the C++ CRC32 guard optional through JAVELIN_EXPECTED_CRC32 and replaces the invalid 0xCRC return literal with named guarded exit codes.
• Documents how to set the expected C++ CRC32 and Python SHA-256 values.

Verification:

• python -m py_compile anti_cheat.py using bundled Python
• python anti_cheat.py exits 0 when no expected hash is set
• matching JAVELIN_EXPECTED_SHA256 exits 0
• mismatched JAVELIN_EXPECTED_SHA256 exits with guarded status 0xC0C

Note: I could not compile the Windows C++ file locally because no C++ compiler is installed in this environment.

[VexCode24]

/claim #4

Demo/verification transcript:

python -m py_compile anti_cheat.py
# passed

python anti_cheat.py
[Javelin AntiCheat] starting checks...
[Javelin AntiCheat] All clear. Continue.

# matching JAVELIN_EXPECTED_SHA256
match rc 0
[Javelin AntiCheat] starting checks...
[Javelin AntiCheat] All clear. Continue.

# mismatched JAVELIN_EXPECTED_SHA256
mismatch rc 3084
[Javelin AntiCheat] Integrity check failed (SHA-256 mismatch). Exiting.

I could not compile the Windows C++ file locally because this environment does not have a C++ compiler installed.