feat(identity): add encode_multikey companion to parse_multikey
Implement encode_multikey to convert AnyVerifyingKey to multibase-multikey
format (base58btc with multicodec curve prefix). This is the inverse of the
existing parse_multikey function and is required for building DID documents
in the self-mint identity server.
Round-trip tests verify both secp256k1 and P-256 keys encode and decode
correctly without information loss.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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