Problem

when a transaction is fetched from an RPC with encoding: json, solana.Message.version is always left as MessageVersionLegacy, even for V0 transactions
The version field is unexported and has no JSON tag, so the default unmarshaler never populates it
This breaks IsVersioned() and any code path that branches on the message version - most notably ALT resolution.

Summary of Changes

• added Message.UnmarshalJSON that detects V0 vs legacy by the presence of the addressTableLookups key
• Message.MarshalJSON perf - emit PublicKeySlice / Hash directly and let their existing fast MarshalJSON methods handle base58 encoding, instead of pre-encoding to []string in a loop
• NewTransaction perf - accountKeyIndex switched from map[string]uint16 (base58-encoded keys) to map[PublicKey]uint16 ([32]byte keys), eliminating one base58 encode per account per instruction

closes #339
related to #262