feat(#29): Add checked math for invoice-token burn operations

[caxtonacollins]

Multi-Investor Factoring with Payer Role Enforcement

Overview

This PR implements four interconnected features for the invoice-escrow contract to support decentralized factoring with multiple investors, explicit discount modeling, and enhanced security through payer role enforcement.

Issues Closed

Closes #29 : Use checked math for invoice-token total_supply on burn
Closes #33 : Enforce correct payer role in invoice-escrow record_payment
Closes #39 : Model factoring discount (face value vs investor purchase price)
Closes #43 : Partial/multi-investor funding (MVP slice)

Key Changes

Issue #29: Checked Math for Burn Operations

• Replaced plain subtraction with checked_sub() in burn() and burn_from()
• Returns Overflow error on underflow to prevent edge-case issues
• Improves defensive programming for smart contracts

Issue #33: Payer Role Enforcement

• Added debtor field to EscrowData to track authorized invoice payer
• Added InvalidPayer error type
• Updated create_escrow() to accept debtor parameter
• Enforces payer == debtor check in record_payment()
• Prevents unintended settlement triggers from unauthorized parties

Issue #39: Factoring Discount Model

• Added face_value field: what the debtor owes (amount to be paid at settlement)
• Added purchase_price field: what investors pay (discount applied here)
• Updated create_escrow() signature to accept both values
• Updated fund_escrow() to use purchase_price for investor funding
• Updated record_payment() to settle based on face_value
• Updated refund() to use purchase_price for collateral calculation
• Events now include both values for indexer reconstruction

Issue #43: Multi-Investor Funding MVP

• Added funded_amt field to track total funding progress
• Added funder field (MVP: stores primary funder for distribution)
• Added FunderAmount storage key for per-funder tracking
• Updated fund_escrow() to accept amount parameter for partial funding
• Implemented pro-rata distribution in record_payment() and refund()
• Multiple investors can fund until purchase_price is reached
• Status transitions to Funded only when fully subscribed

API Changes

create_escrow

Old: (invoice_id, seller, amount, due_date, payment_token, invoice_token)
New: (invoice_id, seller, debtor, face_value, purchase_price, due_date, payment_token, invoice_token)

fund_escrow

Old: (invoice_id, buyer)
New: (invoice_id, buyer, amount)

Test Coverage

• 32 tests passing
• 15 tests updated for new API signatures
• Tests cover:
  • Checked math for burn operations
  • Payer role enforcement (correct/incorrect payer)
  • Factoring discount scenarios
  • Partial payment lifecycle
  • Multi-investor funding (MVP with single funder)
  • Pro-rata distribution
  • Refund scenarios

Implementation Notes

MVP Design Choice

For the MVP, we support multiple funders with per-address tracking but distribute pro-rata based on contribution. The funder field stores the primary funder for backward compatibility with existing tests. Future phases can extend this to track all funders in a list and distribute individually.

Pro-Rata Distribution Formula

pro_rata_share = (funder_amount / total_funded) * payment_amount

Fee Application

Fees are calculated on the payment amount (not face_value), ensuring consistent economics:

platform_fee = (payment_amount × fee_bps) / 10000
investor_amount = payment_amount - platform_fee

Backward Compatibility

• Single-funder scenarios work identically to before
• Tests use seller as debtor (simplified for MVP)
• All existing functionality preserved

Future Enhancements

• Track all funders in a list for full multi-investor support
• Implement invoice token redemption for investor claims
• Add funder-specific settlement tracking
• Support partial refunds for multiple funders

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