Add SHA-256 self-integrity checks

[18166714330cl-maker]

/claim #4

Summary:

• Add SHA-256 self-integrity verification alongside the existing CRC32 check.
• Allow builds to enable CRC32, SHA-256, or both through JAVELIN_EXPECTED_CRC32 and JAVELIN_EXPECTED_SHA256.
• Document the build-time integrity flags in the README.

Testing:

• Not run locally: this source depends on Windows SDK headers (windows.h), and the current environment is macOS without the Windows build toolchain.

Closes #4

[18166714330cl-maker]

Thanks for taking a look.

I treated this as a narrow integrity-check addition rather than changing the anti-cheat flow.

The issue I’m addressing is that CRC32 alone is weak for tamper detection. This PR adds optional SHA-256 verification while preserving the existing CRC32 path and allowing builds to choose CRC32, SHA-256, or both through compile-time flags.

What changed:

• Added SHA-256 self-integrity verification.
• Preserved existing CRC32 behavior.
• Added JAVELIN_EXPECTED_SHA256 documentation alongside the existing CRC32 flag.
• Left the rest of the anti-cheat flow unchanged.

Verification:

• I could not compile locally because this source depends on Windows SDK headers and my environment is macOS.
• I kept the change limited to the integrity-check path and documented the build-time flags.

The main thing I’d like review on is whether the SHA-256 helper placement and flag naming match the project style. If you prefer a different naming convention or build flag shape, I can adjust it.