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

[k-deejah]

closes #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