feat: add 15 genuinely hard prompts for trivial challenge types
- speed_blitz: genesis block nonce, embedded headline, first halving
block height, first pizza transaction value, OP_RETURN payload size
- math_blitz: difficulty adjustment period, sum formula, cross product
magnitude, log2 calculation, secp256k1 field size
- animal_kingdom: octopus hearts, hummingbird flight, shark bones,
giraffe blood pressure, cow stomach count
All tagged difficulty: 'hard' for round 5+ difficulty filtering
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>