feat(nip44): add cross-implementation test vectors and fix ChaCha20 cipher initialization

[Copilot]

Summary

NIP-44 implementation lacked test vectors to verify cross-implementation compatibility with nostr-tools (JavaScript) and contained a critical bug where IvParameterSpec was incorrectly used for ChaCha20 cipher initialization on Java 21+.

Type of change

• fix - Bug fix (non-breaking)
• feat - New feature (non-breaking)
• test - Adding or updating tests

What changed?

Cross-Implementation Test Vectors

Added official NIP-44 test vectors from paulmillr/nip44 repository:

Conversation Key Tests (Nip44ConversationKeyTest.java)

• 18 vectors verify ECDH + HKDF-Extract produces identical keys across implementations
• Edge cases: sec1 = n-2, sec1 = 2, sec1 == pub2 (G point)

Decryption Compatibility Tests (Nip44EncryptDecryptTest.java)

• 10 vectors verify decryption of messages encrypted by JavaScript/Rust/Go implementations
• Unicode coverage: ASCII, emoji, CJK, Arabic, mathematical symbols, kaomoji

Critical Bug Fix

Problem: EncryptedPayloads used IvParameterSpec for ChaCha20, which fails on Java 21:

cipher.init(Cipher.ENCRYPT_MODE, key, new IvParameterSpec(nonce));  // ❌ Invalid

Fix: Use ChaCha20ParameterSpec as required by JCA:

cipher.init(Cipher.ENCRYPT_MODE, key, new ChaCha20ParameterSpec(nonce, 0));  // ✅ Correct

Applied to both encrypt and decrypt paths in EncryptedPayloads.java.

Breaking changes

None. Existing functionality preserved; bug was preventing correct operation on Java 21.

Testing

• Unit tests pass: mvn test
• Integration tests pass: mvn verify
• All 28 new test vectors pass
• No regressions in existing test suite

Review focus

Verify test vectors match official NIP-44 specification at https://github.com/paulmillr/nip44/blob/main/nip44.vectors.json

Checklist

• PR title follows conventional commits: type(scope): description
• Changes are focused and under 300 lines (or stacked PRs)
• Tests added/updated for new functionality
• No new compiler warnings introduced
• CHANGELOG.md updated (for user-facing changes)

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