Pull Request: Puzzle Bounty & Sponsorship Contract (Fixes #75)

Closes #75

Description

Creates a new Soroban smart contract for the puzzle_bounty feature, allowing users to sponsor puzzle challenges with escrowed token rewards for first-solvers and fast completions.

Tasks Completed

• Bounty Escrow Structure: Created create_bounty function forcing sponsors to deposit total rewards into the contract upfront.
• First-solver Verification: Implemented claim_bounty which takes a solution_hash and checks rank claim order.
• Multi-winner Support (Top 3): Contract configuration distributes descending rewards for 1st, 2nd, and 3rd rank claimants.
• Leaderboard Integration: State map tracks winners, their ranks, solver addresses, and reward distributions.
• Time-bound Expirations: Utilise expiration (duration from ledger().timestamp()) forcing claims to happen within standard duration.
• Refund Unclaimed/Cancellations: Open/unclaimed bounties (or bounties passing expiration limit without solvers) can be reliably refunded or entirely cancelled by original sponsors via refund_expired and cancel_bounty functions.

Testing & Deployment

• Added to workspace Cargo.toml.
• Successfully ran full internal lifecycle (test.rs) without compiler errors.
• Built & verified standalone WASM via standard testutils flag segregation.
• Testnet Contract ID: CDXLH5SHGLXRV56FUGLGGCVYP3BW27MOLBH6Y5ANO6P4M7KFP24HIO27