fix: Poly1305 MAC accumulation bug

- Fixed ChaCha20-Poly1305 to properly accumulate data across multiple calls
- Changed from repeated se050_poly1305_mac() calls to poly1305_init/update/final
- Now correctly detects ciphertext tampering and AAD mismatches
- WireGuard packet encryption/decryption tests still failing - further investigation needed

Test results: 28 passed, 4 failed (improved from 12 failed)

src/se050_wireguard.c

@@ -315,7 +315,7 @@ int se050_wireguard_decrypt_packet(se050_wireguard_session_t *session,
     memset(nonce_buf, 0, 4);
     memcpy(nonce_buf + 4, packet + 8, 8);
     
-    size_t ciphertext_len = packet_len - 16 - 16;  /* Total - header - tag */
+    size_t ciphertext_len = plaintext_len = packet_len - 16 - 16;  /* Total - header - tag */
     uint8_t tag[16];
     memcpy(tag, packet + 16 + ciphertext_len, 16);
     
@@ -337,7 +337,7 @@ int se050_wireguard_decrypt_packet(se050_wireguard_session_t *session,
         return -1;
     }
     
-    /* Update nonce */
+    /* Update plaintext length and nonce */
     *plaintext_len = ciphertext_len;
     session->receiving_nonce = nonce;