:sig
String hmac_sha256_hex(String key, String data)

:params
key : secret key
data : message to authenticate
return value : the HMAC-SHA256 as a 64-character lowercase hex string

:content
Computes an HMAC-SHA256 message authentication code and returns it as hex. Use it to sign tokens, cookies, and webhook payloads, then verify with `crypto_equal()` to avoid timing leaks.

:example
print(hmac_sha256_hex("key", "The quick brown fox jumps over the lazy dog"), "\n");

:see
>sys
hmac_sha256
sha256_hex
crypto_equal