test(crypto): add property-based tests for crypto codecs and envelope…

[Anadudev]

Closes #1723

Proposed Changes

Property-Based Testing Suite (tests/unit/crypto/property.test.ts)

• Added a property-based test suite with seedable pseudo-random generators (Mulberry32 PRNG) for byte arrays, hex/base64 strings, and versioned envelopes:
  • Codec Round-Trips: Proved fromHex(toHex(bytes)) === bytes and fromBase64(toBase64(bytes)) === bytes hold for 100+ generated byte arrays of arbitrary length (0 to 512 bytes).
  • Rejection Invariants: Verified that odd-length hex, mixed-case hex, non-hex alphabets, invalid base64 characters, misplaced base64 padding, non-4 base64 lengths, and byte length mismatches strictly throw CodecError.
  • Seal-Open Invariants: Verified that 20 generated valid envelopes pass sealEnvelope -> openEnvelope with exact code point and structure recovery.
  • Tamper Fail-Closed Invariants: Verified that single-byte mutations to nonces, MACs, ciphertext, content commitments, or recipient keys strictly fail closed across generated envelopes.
  • Reproducible Seed Reporting: Ensured any assertion error reports the exact random seed (seed: ${seed}) for deterministic bug reproduction.

Verification Plan

Automated Tests

• Executed node node_modules/vitest/vitest.mjs run tests/unit/crypto/property.test.ts: 5 property tests passed in 140ms.
• Executed node node_modules/vitest/vitest.mjs run tests/unit/crypto: 27 test files passed, 245/245 tests passed.
• Executed npx eslint tests/unit/crypto/property.test.ts: 0 lint errors.