Remove mixed computation metadata from variables

[MaxGhenis]

Summary

• remove redundant uprating metadata from variables already computed by adds
• move aggregation lists used by formulas to module constants so variables no longer declare both formula and adds/subtracts
• keep a changelog fragment so this cleanup releases before PolicyEngine Core enforces mutually exclusive computation modes

Verification

• AST scan for variables mixing formula, adds/subtracts, and uprating: conflicts=0
• uv run --no-sync ruff check on touched variable files
• uv run --no-sync ruff format --check on touched variable files
• uv run --no-sync python -c "from policyengine_uk import CountryTaxBenefitSystem; CountryTaxBenefitSystem(); print('uk import OK')"
• PYTHONPATH=/Users/maxghenis/policyengine-core uv run --no-sync python -c "from policyengine_uk import CountryTaxBenefitSystem; CountryTaxBenefitSystem(); print('uk import OK with local core')"
• uv run --with towncrier towncrier build --draft --version 2.88.18

Companion cleanup for US: PolicyEngine/policyengine-us#8330. Core guard PR to follow after UK and US releases are live.