feat: Add SilverCents demo - silver-backed offchain cash system on Basis

[whitks]

DONE BY TEAM QWERTY:
I have created a cli: these are the main features; Rest of the details have been included in silvercents.md file

• deposit: Record physical silver into offchain reserve ledger
• issue: Create SilverCent IOU notes for recipients
• pay: Transfer SilverCents between participants (offchain)
• redeem: Burn notes and withdraw from on-chain reserve
• status: View reserve collateralization and outstanding supply

PR Description

Overview

This PR introduces SilverCents, a complete demonstration of the Basis offchain cash system applied to real-world commodity-backed community currencies. SilverCents are offchain IOU notes issued under the Basis model that are redeemable against on-chain reserves backed by physical U.S. constitutional silver coins (dimes and quarters).

What is SilverCents?

SilverCents showcases how Basis can power decentralized, commodity-backed cash systems:

• Offchain notes: Lightweight IOU tokens representing claims on physical silver
• Reserve-backed: On-chain Ergo smart contract maintains collateralization
• Community currency: Demonstrates real-world applicability beyond traditional crypto
• Complete lifecycle: Issuance → circulation → redemption

Key Components

1. CLI Commands (crates/basis_cli)

• deposit: Record physical silver into offchain reserve ledger
• issue: Create SilverCent IOU notes for recipients
• pay: Transfer SilverCents between participants (offchain)
• redeem: Burn notes and withdraw from on-chain reserve
• status: View reserve collateralization and outstanding supply

2. Offchain Reserve Ledger

• JSON-based tracking of physical silver reserves
• Maintains: total silver, issued notes, redeemed amounts
• Enables collateralization ratio calculations
• No blockchain required for tracking (pure offchain)

3. On-Chain Integration

• Uses existing Basis reserve contract on Ergo
• Enforces redemption against tracked collateral
• Records on-chain settlement of offchain claims
• Supports multi-signature governance (optional)

4. Documentation

• Complete economic model explanation
• CLI usage examples with real output
• Testing procedures and prerequisites
• Limitations and future extensions clearly documented

Technical Details

Economic Model

1. Backing Asset: U.S. constitutional silver dimes/quarters held offchain
2. Peg: 1 SilverCent = 1 physical silver coin (demo assumption)
3. Issuance: Backed by declared physical reserves
4. Redemption: Holders can redeem notes against on-chain collateral