CryptoKey structured serialization was cloning the internal algorithm
slot as a generic JS object. After deserialization, WebCrypto operations
such as exportKey() expected the slot to still be a native KeyAlgorithm
subclass and crashed when downcasting it.
Serialize the stored key algorithm with an explicit tag and reconstruct
the appropriate KeyAlgorithm subclass during deserialization. Also keep
EC private-key serialization aligned by encoding nested public keys as
EC public-key payloads rather than tagged top-level handles.
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