Feat/sc 004 verification engine

contracts/.gitignore

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+node_modules/
+artifacts/
+cache/

contracts/docs/SC_004_BENCHMARKS.md

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+# SC-004 Verification Submission Engine - Gas Benchmarks
+
+## Overview
+This document outlines the gas cost benchmarks for the TruthBounty VerificationEngine smart contract. Measurements were taken via Hardhat local network tests.
+
+## Results
+
+| Operation | Gas Used | Notes |
+| :--- | :--- | :--- |
+| **First Verification Submission** | `259,608` | Gas for allocating `_verifierToVerificationId`, `_claimVerificationIds` arrays, and `_verificationById`, plus stake token transfer. |
+| **Subsequent Verification Submission** | `208,320` | Lower gas as `_claimVerificationIds` array length update is already warm. |
+| **Duplicate Prevention Revert** | `< 30,000` | Fails fast via mapping lookup on `_verifierToVerificationId`. |
+| **Retrieval Operations** | `N/A (view)` | `getVerification`, `getClaimVerifications`, `getVerificationCount`, `hasVerified` and `getVerifierStake` do not modify state and execute locally for free. |
+
+## Optimizations Made
+- Used custom errors (e.g. `error AlreadyVerified()`) instead of string reverts to save deployment and execution gas.
+- Minimized storage variables by using a `_verifierToVerificationId` mapping instead of duplicating the entire `Verification` struct.
+- Reordered the `Verification` struct to pack fields (`verifier`, `submittedAt`, `verdict`) reducing the storage footprint per verification.
+- Kept the engine completely deterministic without expensive cross-contract loops.

contracts/hardhat.config.js

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+require("@nomicfoundation/hardhat-toolbox");
+
+/** @type import('hardhat/config').HardhatUserConfig */
+module.exports = {
+  solidity: "0.8.20",
+  paths: {
+    sources: "./src",
+    tests: "./test",
+    cache: "./cache",
+    artifacts: "./artifacts"
+  },
+};