Comprehensive test suite covering:
- Mint and validate with bits=2
- Replay detection
- Resource mismatch
- Invalid format, bad version, bad date
- Insufficient difficulty
- Expired stamps
- Zero bits bypass
- Long date format (YYMMDDHHMMSS)
- Multiple unique stamps
- Higher difficulty stamps accepted at lower threshold
Move the hashcash proof-of-work stamp from the X-Hashcash HTTP header
into the JSON request body as a 'hashcash' field on POST /api/v1/session.
Updated server handler, CLI client, SPA client, and documentation.