feat: implement circuit breaker lock in Soroban contract

[Jennylila]

Overview

This PR implements a per-market circuit breaker for the Soroban prediction market contract to protect pools from anomalous betting activity. It adds persistent circuit-breaker state and rolling pool snapshots, blocks new bets once the breaker is triggered, emits an on-chain event for indexers, and provides admin recovery flows to reopen or force-resolve affected markets.

Related Issue

Closes #143

Changes

⚙️ Circuit Breaker Implementation

• [MODIFY] contracts/prediction_market/src/lib.rs
  • Added DataKey::CircuitBreaker(u64) and DataKey::PoolSnapshot(u64) to contract storage.
  • Added a persistent circuit-breaker flag per market.
  • Added a persistent rolling pool snapshot per market.
  • On each place_bet, compares the current pool against the snapshot from at least 60 seconds earlier.
  • Triggers the circuit breaker when pool movement is greater than 50% within the 60-second window.
  • Rejects later bets with CIRCUIT_BREAKER_ACTIVE once the breaker is active.
  • Emits a breaker contract event with market ID, previous snapshot, current pool, and trigger timestamp.
  • Refreshes the stored pool snapshot using ledger timestamp checks.

🛠️ Admin Recovery Functions

• [MODIFY] contracts/prediction_market/src/lib.rs
  • Added reopen_market(market_id) to clear the breaker and refresh the snapshot baseline.
  • Added force_resolve(market_id, outcome) to allow emergency admin resolution.
  • Both recovery functions require admin auth.

🧪 Test Coverage

• [MODIFY] contracts/prediction_market/src/lib.rs
  • Added unit tests for:
    • breaker default state
    • trigger when movement exceeds 50% after 60 seconds
    • no trigger at exactly 50%
    • no trigger inside the 60-second window
    • bets blocked after trigger
    • admin reopen flow
    • admin force-resolve flow
  • Verified the full contract test suite passes.

📚 Documentation

• [MODIFY] README.md
  • Documented circuit breaker states.
  • Documented the admin recovery procedure.
  • Described how the 60-second snapshot and trigger threshold work.

📦 Dependency / Audit Maintenance

• [MODIFY] contracts/prediction_market/Cargo.lock
  • Updated lockfile dependencies so cargo audit passes without high or critical advisories.

Verification Results

Acceptance Criteria	Status
Circuit breaker triggers correctly when pool movement exceeds 50% within 60 seconds	✅
New bets are rejected with CIRCUIT_BREAKER_ACTIVE after trigger	✅
Contract event is emitted on trigger	✅
reopen_market and force_resolve require admin auth and work correctly	✅
Unit tests cover trigger and recovery edge cases	✅
cargo audit passes with zero high or critical advisories	✅
README documents breaker states and recovery procedure	✅

How to Test

# 1. Confirm you're on the feature branch
git branch --show-current

# 2. Run the contract tests
cd contracts/prediction_market
cargo test
<img width="808" height="393" alt="image" src="https://github.com/user-attachments/assets/f6fd42a1-2c79-4b11-98ae-eb1dbb921989" />

# 3. Verify dependency audit
cargo audit

# 4. Optional: inspect the main changed files
git diff -- README.md contracts/prediction_market/src/lib.rs contracts/prediction_market/Cargo.lock

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