fix(crypto): implement crypto primitives, hex wire format, and full test suite - #48

…est suite (AnonVote#39)

- Add DECISIONS.md documenting ADR-001: hex output format for encryptVote
- Implement encryptVote returning EncryptedPayload object with hex-encoded
  ciphertext, iv, and authTag (fixes base64/hex mismatch with AnonVote/core)
- Implement decryptVote accepting EncryptedPayload; auth tag failure throws
  AnonVoteCryptoError(DECRYPTION_FAILED) — never swallowed silently
- Validate key as exactly 64 hex chars; throw AnonVoteCryptoError(INVALID_KEY)
  before any cipher op
- Validate payload fields in decryptVote; throw INVALID_PAYLOAD if missing/empty
- hashIdentifier: trim + lowercase normalisation enforced
- generateToken: 32-byte CSPRNG, hex output, JSDoc @warning against reuse
- hashToken: separate export from hashIdentifier for independent testability
- Add EncryptedPayload, Token, Vote, ElectionResult, BallotEvent interfaces
  and AnonVoteCryptoError class to src/types.ts
- Export all canonical types and AnonVoteCryptoError from src/index.ts
- Rewrite tests/crypto.test.ts with all 25 cases per issue spec
- Update src/client.ts and tests/client.test.ts to use new EncryptedPayload
  object (serialised as JSON string in VoteReceipt.encryptedPayload)
- All 73 tests pass; clean tsc build with no errors