Add gas benchmarking and optimization suite for contract functions

[Smartdevs17]

Description

Implement a comprehensive gas benchmarking framework that tracks gas costs for all contract functions and alerts on regressions.

Acceptance Criteria

• Gas benchmark for every public function
• CI gate on gas increase >10%
• Historical gas trend dashboard
• Gas optimization suggestions based on patterns
• Storage slot usage analysis
• Cross-contract call gas tracking

Technical Scope

• stellar-lend/contracts/hello-world/src/gas_benchmark.rs
• stellar-lend/benchmarks/
• .github/workflows/
• Edge: benchmark result variance, storage slot measurement, cross-contract accounting