Add Trade Credit & Invoice Factoring Extensions (B2B analytics)

[Thorium]

Summary

This PR introduces the initial Business (B2B) analytical extensions under the namespace FSharp.Finance.B2B.*.

Purpose: provide optional analytics (NOT a full commercial / operational platform) for:

• Trade credit early‑payment discount implied annual rates
• Basic invoice factoring parameter construction
• Lightweight product metadata classification
• Forward‑looking generic cashflow scaffolding (not yet integrated)

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Files Added (Current State)

• DomainExtensions.fs
  • BusinessProductType DU
  • ProductMetadata record + helper constructors (consumer, tradeCredit, invoiceFactoring)
• CashflowModel.fs
  • CashflowType DU + CashflowEvent record (future-facing abstraction; not yet wired in)
• TradeCredit.fs
  • DiscountTerms, createTerms
  • impliedAnnualRateSimple
  • impliedAnnualRateCompounded
• InvoiceFactoring.fs
  • Invoice, InvoiceAdvance (derive)
  • FactoringParameters.build → (Parameters * InvoiceAdvance list)
• Docs: docs/exampleTradeCreditAndFactoring.fsx
• README: B2B section added

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Design Decisions

Aspect	Decision
Namespace boundary	All new code under FSharp.Finance.B2B.*
APR handling	Reuse Apr.CalculationMethod.UnitedKingdom 2 (placeholder)
Factoring modeling	Single repayment per invoice at face value (advance + reserve implicit)
Product metadata	Non-regulatory, descriptive only
Cashflow model	Present but not integrated yet (reserved for future factoring / reserves modeling)

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Trade Credit Rate Formulas

Let d = discountPercent, D = discount deadline day, N = net due day.

Simple annualized (no intra-year compounding):
rate_simple = (d / (1 - d)) * (365 / (N - D))

Naive compounded:
rate_compounded = (1 + d / (1 - d))^(floor(365 / (N - D))) - 1

Comments in code document rationale and limitations.

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Documentation Example

docs/exampleTradeCreditAndFactoring.fsx:

• Demonstrates “2/10 net 30” implied rates
• Builds factoring parameters for multiple invoices
• Prints schedule basics & economics
• Includes disclaimers

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Tests (Planned / Needed)

• TradeCreditTests.fs
  • Assert impliedAnnualRateSimple for 2/10 net 30 ≈ 0.3724 (tolerance ±0.0001m)
• (Optional follow-up) factoring structural integrity test

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Checklist

• Add B2B source modules
• Update README
• Add documentation example
• Add trade credit unit test

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Open Questions

1. Should this be it's own nuget package, because it's not "Personal", however the same business shares the benefit of current amortisation.
2. Introduce AprMethod.Disabled in a future minor version for zero-interest cases?
3. Explicit reserves as separate modeled cashflows later (integrating CashflowModel)?
4. Future consolidation into a neutral FSharp.Finance.Analytics namespace (backward-compatible duplication)?

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Non-Goals (Current Iteration)

• Operational factoring workflows (collections, dilution)
• Risk / expected loss modeling
• Multi-currency support
• Regulatory classification

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Disclaimer

Analytical utilities only. Independent validation required for production or compliance use.