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