feat(gas-evaluator): implement real-time gas and fee delta estimation engine

[Vicky08100]

Closes #588

PR Description

I implemented a real-time gas and fee delta estimation engine for Solidity/EVM and Stellar/Soroban contracts. This tool helps developers assess the exact monetary savings of applying optimizations by evaluating gas differences ($\Delta$Gas) in USD and native gas tokens (ETH and XLM).

I created a model that maps Solidity optimization rules to EVM opcode gas differentials (like SLOAD cold vs warm, checked math vs unchecked block, call caching, logging, and loop limits). I also built a Soroban resource calculator that maps rules to Stellar CPU instructions and Ledger read/write byte costs according to the 2026/Protocol 20+ schedules.

To convert gas/resource units into USD, I integrated a cached pricing service that retrieves current ETH and XLM prices from CoinGecko, caches the values via the Redis CacheService, and handles connection fallbacks gracefully. Using historical transaction execution volume, the engine estimates the annual native token and USD savings.

Changes Made

• Created the @gasguard/gas-evaluator package inside libs/gas-evaluator/
• Implemented libs/gas-evaluator/src/evm-opcode-calculator.ts for mapping Solidity optimization rules to EVM opcode gas differentials
• Implemented libs/gas-evaluator/src/soroban-resource-calculator.ts for mapping Soroban rules to Stellar CPU and Ledger byte schedules
• Implemented libs/gas-evaluator/src/gas-delta-evaluator.ts to orchestrate calculation flow, fetch token prices, cache them, and compute annual savings
• Added export entry points in libs/gas-evaluator/src/index.ts
• Updated root tsconfig.json with path mapping for @gas-evaluator/*
• Updated root package.json Jest config with moduleNameMapper path alias
• Added unit test suite in libs/gas-evaluator/tests/gas-delta-evaluator.spec.ts

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