Tax Engine — § 25 UStG Steuer-Engine

Implementation stub. Full specification lives in the Tax Engine Protocol.

Module Structure

apps/api/src/
├── backoffice/
│   └── services/
│       └── tax-resolver.service.ts     # EIGEN/FREMD + Geography → tax_strategy
├── commerce/
│   └── services/
│       ├── tax-calculator.service.ts   # Actuals → TaxLedgerEntry (§ 25 Abs. 5 fields)
│       ├── invoice.service.ts          # TaxLedgerEntry → Invoice
│       └── period-lock.service.ts      # Immutability enforcement
└── shared/
    └── tax/
        ├── tax-strategy.enum.ts        # STANDARD_VAT | MARGIN_SCHEME_25
        ├── geography.enum.ts           # EU | THIRD_COUNTRY
        ├── schemas.ts                  # Valibot: CostComponent, TaxRule, MarginResult
        ├── margin-calculator.ts        # § 25 UStG: dynamic rate, split, Negativmarge
        └── vat-calculator.ts           # Standard VAT: configurable rate