Performs sign-then-encrypt operation: signs data with signKid, then encrypts
the resulting JWS with encryptKid to produce a nested JWE(JWS) structure.

Both keys must be homogeneous (both symmetric or both asymmetric). Both keys
must be different for security.

Supports SIMPLE keys (Compact JWE/JWS) and COMPOSITE keys (General JSON Serialization).
COMPOSITE keys enable hybrid post-quantum cryptography with dual signatures (COMPOSITE_SIGNATURE)
and combined key encapsulation (COMPOSITE_KEM_COMBINE).

Key type combinations:

• Symmetric + Symmetric (e.g., AES-GCM + HMAC-SHA256)
• Asymmetric + Asymmetric (e.g., ML-KEM + ML-DSA, RSA + ECDSA)
• COMPOSITE + COMPOSITE (e.g., COMPOSITE_KEM_COMBINE + COMPOSITE_SIGNATURE)
• COMPOSITE + SIMPLE (any homogeneous combination)

The JWE outer header includes cty="JWS" to indicate nested content.