Add decoding for Monopoly reward keys and switch reward metadata to be looked up by decoded id. Implement decode_reward_key() and infer_reward_type(); replace the previous raw-key KNOWN_REWARDS approach with REWARDS_BY_ID built from mappings/*.json. Add mappings/items.json and expand mappings/arcs.json. Update decoder to produce reward_id, use REWARDS_BY_ID, and change guess_quantity() to accept reward_id. Refactor pool_mappings to reuse the new mapping loader, and update tests and docs to validate mapping files and document the reward key encoding/behavior.
