# Additions
- `implementation/reference/web4_crypto_stub.py`: toy COSE/JWS facade (interfaces stable; crypto dummy).
- `schemas/problem-details.schema.json`: JSON Schema for Problem Details.
- `implementation/tests/test_negative_replay.py`: timestamp/replay window tests.
- `implementation/tests/test_negative_downgrade.py`: downgrade detection test.
- `artifacts/handshake_state_machine.svg`: static render of the core state machine.
