feat: implement CrowdFunding Vending Machine smart contract (fixes #7)

[s6pa1rta3n-lab]

Summary

This pull request implements the CrowdFunding Vending Machine smart contract for presale and crowdfunding campaigns as specified in #7.

Key Capabilities & Implementation

• Campaign Creation (createCrowdCampaign): Enables creators and organizations to launch crowdfunding campaigns specifying a product metadata URI, funding goal, unit price, campaign duration, and max supply.
• Supporter Pledges (supportCampaign): Backers purchase and reserve allocations using ETH. Validates exact payments, supply bounds, and deadlines.
• Product Redemption (redeemProduct): Once the funding goal is reached, contributors can redeem their purchased quantities.
• Creator Settlement (claimFunds): Campaign managers can withdraw raised proceeds once goals are satisfied.
• Cancellation & Backer Refunds (cancelCampaign, refund): If a campaign expires without reaching its target or is cancelled by the creator, contributors can withdraw full ETH refunds via a pull-based pattern.
• Inspection & Helpers: Complete helper views (getCampaign, stateOf, getSupporterInfo) for integrations and UI/subgraph querying.
• Security & Reliability: Implements ReentrancyGuard, checks-effects-interactions pattern, custom errors for gas efficiency, and comprehensive event emissions.

Verification

• 32/32 tests passing locally via Hardhat (npx hardhat test).
• Zero mocked assertions — full state transition and real balance validation on local Ethereum network.

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC