Thread safety

All handle classes (Account, OlmSession, OutboundGroupSession, InboundGroupSession, Sas, EstablishedSas, Ecies, EstablishedEcies, Ed25519KeyPair, PkEncryption, PkDecryption) are safe to use from multiple threads. Methods that access the native state are synchronized on the instance, and close() can never race with an ongoing native call.

• Sharing a single handle across threads serializes the native calls on that object.
• Distinct handles can be used concurrently without any locking.
• InboundGroupSession.connected/compare/merge lock both operands in deterministic order to prevent deadlocks.

Value classes (MegolmMessage, OlmMessage, IdentityKeys, result types, key types) are immutable and can always be shared freely.