Add emergency circuit breaker with tiered triggers

[Smartdevs17]

Description

Implement an emergency circuit breaker system that can pause protocol operations at different tiers (single pool, all pools, entire protocol) based on severity.

Acceptance Criteria

• Tier 1: Single pool pause (oracle issues, pool manipulation)
• Tier 2: All lending paused (systemic risk detected)
• Tier 3: Full protocol halt (critical vulnerability)
• Automatic trigger conditions (price deviation >20%, abnormal utilization)
• Guardian multisig for manual trigger
• Governance vote required for Tier 3 unpause

Technical Scope

• stellar-lend/contracts/hello-world/src/circuit_breaker.rs
• stellar-lend/contracts/lending/src/
• Edge: false trigger during high volatility, circuit breaker DoS, governance vs guardian authority