feat(zk): add ElGamal & AES key derivation

[sonicfromnewyoke]

Problem

solana-go has no Token-2022 confidential-transfer key-derivation support #412. Rust solana-zk-sdk and JS @solana/zk-sdk ship deterministic KDFs that turn a Solana signer + public seed into an ElGamal secret scalar and an AES-128-GCM-SIV key;
Go users currently have to call into Rust or JS/WASM just to produce the key material a confidential transfer needs.
First step of the port is the KDF itself, byte-for-byte compatible with the reference SDKs.

Summary of Changes

• AeKey (16 bytes, AES-128-GCM-SIV key): AeKeyFromSeed, AeKeyFromSignature, AeKeyFromSigner, AeKeyFromSeedPhraseAndPassphrase
• ElGamalSecretKey (32 bytes, canonical Ristretto/Ed25519 scalar mod ell): ElGamalSecretKeyFromSeed, ElGamalSecretKeyFromSignature, ElGamalSecretKeyFromSigner, ElGamalSecretKeyFromSeedPhraseAndPassphrase
• Signer interface (minimal: Sign(message) -> solana.Signature, error); solana.PrivateKey satisfies it; hardware wallets and remote signers can plug in without changes.
• Seed-length bounds and default-signature rejection mirror the Rust implementation (ErrSeedTooShort, ErrSeedTooLong, ErrDefaultSignature).

related to #412