Implement UK situation builder

[MaxGhenis]

Summary

Create a UK-specific situation builder function similar to create_situation() for US.

Implementation

Create givecalc/uk/situation.py with:

• create_uk_situation() function that builds a PolicyEngine-UK compatible situation dictionary
• Support for key UK inputs:
  • employment_income (wages/salary)
  • self_employment_income
  • gift_aid (main charitable donation mechanism)
  • charitable_investment_gifts (donations of stocks/property)
  • is_married (affects tax calculations)
  • region (England, Scotland, Wales, NI - affects income tax rates)
  • num_children (affects child benefit)
• Create donation axes (sweep gift_aid from 0 to income)
• Use UK entity structure: Person → BenUnit → Household

UK Entity Mapping

{
    "people": {"person": {...}},
    "benunits": {"benunit": {"members": ["person"]}},
    "households": {"household": {"members": ["person"]}}
}

Acceptance Criteria

• TDD: Tests written first
• create_uk_situation() creates valid PolicyEngine-UK situations
• Axes correctly sweep gift_aid donations
• Handles single person and married couple scenarios
• Handles all 4 UK nations

Part of #59