#159 Add storage TTL management with automatic instance and persistent state extension

[webdevayo]

Summary

Implements full Storage TTL (Time-To-Live) management across all smart contracts to prevent on-chain state from expiring. This ensures balances, allowances, lockups, and instance storage remain valid as long as the contract is actively used.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 📝 Documentation update
• 🔧 Smart contract improvement
• 🧪 Test coverage improvement
• 🏗️ CI/Build improvement

Related Issue

Closes #159

Changes Made

• Implemented centralized TTL management for instance and persistent storage
• Extended instance TTL on every public contract entry function
• Added TTL refresh for balances, allowances, and lockup storage
• Introduced extend_balance_ttl(address) for user-level balance TTL extension
• Added extend_ttl() for instance-level TTL management
• Ensured expired balances and allowances safely return 0 instead of failing
• Added lockup TTL handling for lock and withdraw operations
• Added regression tests for TTL persistence and recovery scenarios
• Verified all public functions manually for TTL injection coverage via static inspection (grep + code walkthrough)

Testing

How has this been tested?

• Rust unit tests pass (cargo test) — executed via CI due to local Cargo registry/DNS limitations
• SDK compiles (npm run build in sdk/)
• Manual testing against Soroban testnet
• New tests added for changes

Test commands run:

# Contract tests (run in CI due to local network limitations)
cargo test

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