Add firm-level VAT implementation

[MaxGhenis]

Summary

Implements firm-level VAT logic for PolicyEngine UK, enabling modeling of VAT at the business level with registration thresholds and rate calculations.

Changes

• ✨ Added Firm entity with owner role linking to persons
• 📊 Created VAT parameters:
  • Registration threshold (£90,000)
  • Deregistration threshold (£88,000)
  • Zero rate (0%)
• 🧮 Implemented 8 VAT calculation variables:
  • firm_turnover: Annual business turnover
  • firm_vat_registered: Registration status based on threshold
  • firm_standard_rated_supplies: Supplies at 20% rate
  • firm_reduced_rated_supplies: Supplies at 5% rate
  • firm_zero_rated_supplies: Supplies at 0% rate
  • firm_vat_on_sales: Output VAT calculation
  • firm_vat_on_purchases: Input VAT
  • firm_net_vat_liability: Net VAT (output - input)

Testing

• ✅ 8 tests passing (3 entity tests, 5 VAT tests)
• 🧪 Tests cover entity creation, VAT thresholds, and calculations
• 🎨 Code formatted with Black

Use Cases

This enables PolicyEngine to:

• Model VAT policy changes on businesses
• Analyze VAT threshold impacts
• Calculate VAT revenue from firms
• Support business tax reform analysis

Fixes #1320

🤖 Generated with Claude Code

[MaxGhenis]

The test failure appears to be unrelated to the firm VAT implementation. The error is an ImportError with the 'tables' module in test_reform_impacts.py:

ERROR collecting policyengine_uk/tests/microsimulation/test_reform_impacts.py
ImportError: Missing optional dependency 'pytables'

All the firm VAT tests passed successfully (5 tests in test_firm_vat.py). This seems to be an existing CI issue with the pytables dependency on macOS.

[MaxGhenis]

CI Status Update

The firm-level VAT implementation is structurally complete but faces integration challenges with the existing test infrastructure:

What's Implemented ✅

• Firm entity definition
• VAT parameters (registration/deregistration thresholds, rates)
• 10 VAT calculation variables (turnover, registration status, various rated supplies, VAT on sales/purchases, net liability)
• Firm weight variables for microsimulation
• Unit tests for firm VAT calculations (passing locally when run in isolation)

Integration Challenge ⚠️

The core issue is that adding the Firm entity to the tax-benefit system causes YAML-based tests to fail because:

1. YAML tests don't provide firm population data
2. The core Simulation class tries to check all variables including firm ones
3. This creates a ValueError when accessing firm populations that don't exist

Attempted Solutions

1. ✅ Added pytables to dev dependencies (fixed HDF5 import issue)
2. ✅ Added firm_weight and firm_count variables (fixed microsimulation weight issue)
3. ⚠️ Tried making firm populations optional - causes our firm tests to fail
4. ⚠️ Tried removing Firm from default entities - breaks firm simulations
5. ⚠️ Tried dynamically adding Firm entity - complex interaction with core Simulation class

Next Steps

This PR demonstrates the implementation approach for firm-level VAT. To fully integrate, we would need to either:

• Modify policyengine-core to handle optional entities gracefully
• Create a separate test runner for firm-specific tests
• Refactor how entities are loaded to be more dynamic

The code is ready for review as a proof of concept for firm-level taxation modeling.

[MaxGhenis]

@nikhilwoodruff @vahid-ahmadi could you take a look to see how we can pass the tests? Adding a new entity type seems to cause some issues.

[MaxGhenis]

Superseded by #1332