Implements OP_OPENSSL_SHA256 to compute a SHA-256 hash in hex.
Behavior:
- Pops one value from the VM stack and converts it to bytes by first producing its string form with value_to_string_alloc().
- Computes the SHA-256 digest via fun_openssl_sha256_hex() and pushes the lowercase hexadecimal string.
- On allocation or hashing failure, pushes an empty string ("").
Notes:
- Non-string inputs are allowed; values are stringified first.
- Snippet is included by vm.c and executed for OP_OPENSSL_SHA256.
Errors:
- No hard VM error is raised; failures push an empty string.
Definition in file sha256.c.