Implement UK tax calculations

[MaxGhenis]

Summary

Create UK-specific tax calculation functions similar to US calculate_donation_effects().

Implementation

Create givecalc/uk/tax.py with:

• calculate_uk_donation_effects() - Calculate tax impact across donation range
• calculate_uk_donation_metrics() - Calculate metrics at specific donation amount
• create_uk_donation_dataframe() - Build DataFrame with analysis results

Key UK Tax Variables

• income_tax: Total income tax liability
• household_benefits: Total benefits received
• household_net_income: Market income + benefits - taxes
• gift_aid: Charitable donations via Gift Aid

UK Gift Aid Mechanics

Gift Aid provides tax relief in two ways:

1. Charity reclaim: Charity claims 25p per £1 donated (basic rate relief)
2. Donor relief: Higher/additional rate taxpayers claim additional relief

For a £100 donation by a 40% taxpayer:

• Charity receives: £100 + £25 (basic rate reclaim) = £125
• Donor claims: £25 (difference between 40% and 20% on £125 gross)
• Net cost to donor: £100 - £25 = £75
• Total charity benefit: £125
• Total government subsidy: £50 (£25 to charity + £25 to donor)

Acceptance Criteria

• TDD: Tests written first
• Correctly calculates UK income tax for various income levels
• Gift Aid donations reduce taxable income appropriately
• Marginal savings rate calculated correctly
• Higher rate relief shown for 40%+ taxpayers

Part of #59