Small ECDH envelope helper.
Keys are raw elliptic-curve key material encoded with Base64. Payload encryption uses
KMS.Crypto with a key derived from the ECDH shared secret.
Envelopes derive AES-GCM keys with HKDF-style context binding. :key_size (or
:symmetric_key_size) may be 16, 24, or 32 bytes and defaults to 16 bytes for
compatibility. :info should identify the caller's envelope purpose and stable
resource context.
The default :legacy KDF is preserved for existing envelopes. Callers may opt in
to kdf: :hpke_labeled for RFC 9180-inspired labeled extract/expand domain
separation. This is HPKE-aligned ECDH envelope encryption, not a complete RFC
9180 HPKE implementation.