feat: add Std::Crypto — SHA-256, HMAC-SHA256, Base64, random bytes

- C runtime (OpenSSL): bux_sha256, bux_hmac_sha256, bux_random_bytes,
  bux_base64_encode, bux_base64_decode, bux_bytes_to_hex
- library/std/Crypto.bux: Crypto_Sha256, Crypto_HmacSha256,
  Crypto_HmacSha256Raw, Crypto_RandomBytes, Crypto_Base64Encode,
  Crypto_Base64Decode
- examples/jwt.bux: full JWT creation with HS256 signature
- Link with -lcrypto in C backend

_test_crypto/bux.toml

@@ -0,0 +1 @@
+[Package]

_test_crypto/src/Main.bux

@@ -0,0 +1,18 @@
+import Std::Io::{PrintLine};
+import Std::Crypto;
+
+func Main() -> int {
+    PrintLine("SHA256:");
+    PrintLine(Crypto_Sha256("hello"));
+    
+    PrintLine("HMAC-SHA256:");
+    PrintLine(Crypto_HmacSha256("secret", "message"));
+    
+    PrintLine("Base64 encode:");
+    PrintLine(Crypto_Base64Encode("hello"));
+    
+    PrintLine("Random bytes (base64):");
+    PrintLine(Crypto_RandomBytes(16));
+    
+    return 0;
+}