feat: Implement clawback health check for regulated assets (Issue #200)

[Xhristin3]

• Add DeficitRecord and ClawbackBuffer structures for tracking clawback events
• Add SafetyPaused status to GrantStatus enum
• Add deficit tracking fields to Grant struct (deficit_detected, deficit_amount, etc.)
• Implement perform_clawback_health_check() to verify contract balance before withdrawals
• Implement withdraw_proportional_during_safety_pause() for fair distribution during deficit
• Add admin functions: resolve_deficit_with_treasury_funds(), activate_safety_pause(), deactivate_safety_pause()
• Add getter functions: get_deficit_record(), get_clawback_buffer()
• Emit DeficitDetected event when contract balance < total owed
• Automatically trigger safety pause to prevent race conditions on clawback
• Allow proportional withdrawals after 4-hour buffer period expires
• Add comprehensive integration tests for clawback health check scenarios

Security: Prevents first-withdrawer advantage during external clawback events
Compliance: Handles regulated assets with clawback-enabled tokens gracefully

closes #103

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