feat(draws): fix #30 verifiable unbiased commit-reveal draws

[fhayvy]

Closes #30

Summary

Implemented a production-grade, verifiable, and manipulation-resistant commit-reveal draw mechanism in the drip-pool Soroban smart contract to replace the deterministic/admin-only placeholder draw winner selection.

Key Technical Enhancements:

• Commit-Reveal Mechanism: Initiates draws in two steps (commit_draw and finalize_draw) to prevent admins or providers from predicting or skewing outcomes before commitment.
• Freeze Boundary Lock: Blocks all joins, deposits, and withdrawals while a committed draw is active. This naturally freezes eligible participants and weights at the freeze_ledger boundary without requiring expensive storage snapshots.
• Verification & Completeness Enforcement: Requires the caller of finalize_draw to submit a complete participant address list whose deposit weights sum exactly to the pool's total deposited balance, preventing provider-side omission bias.
• Domain Separation: Combines network passphrase hash, contract address, round ID, freeze sequence, and the revealed secret to generate a secure random seed.
• Modulo Bias Mitigation: Maps the seed to the winners using a mathematical rejection sampling loop.
• Liveness & Timeout Fallbacks: Implements a time-bounded deadline (reveal_deadline) after which anyone can call cancel_draw to unlock the pool and allow normal participant operations to resume.

[BigDella]

@fhayvy fix the error

[BigDella]

@fhayvy fix merge conflict

[fhayvy]

@BigDella

[BigDella]

@fhayvy great job