Remove pass-through allowance variables (#414)

[vahid-ahmadi]

Summary

• Removes trading_allowance, property_allowance, and dividend_allowance — three Variables whose formulas only returned the same-named parameter value. Callers (trading_allowance_deduction, property_allowance_deduction, taxable_property_income, taxed_dividend_income) now read those parameters directly.
• Fixes the taxable_self_employment_income_deductions parameter list to reference trading_allowance_deduction (the amount capped at self_employment_income) rather than the raw trading_allowance. Pre-existing max_(0, ...) meant outputs were already equivalent; this makes the intent explicit.
• Moves tests that injected these variables as inputs to override the corresponding parameter path instead. Drops the now-meaningless test that asserted the default dividend_allowance value.

Closes #414.

Test plan

• policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/income_tax -c policyengine_uk — 9 failed / 145 passed, identical 9 preexisting failures on main (gift-aid/PA taper, pension annual allowance, marriage allowance); one fewer pass corresponds to the deleted dividend_allowance output assertion.
• Full CI run (make test) on this branch

🤖 Generated with Claude Code

[MaxGhenis]

Holding this one because a first-party data pipeline still emits two of the variables being removed. policyengine-uk-data currently sets \ and \ in ; after this PR those names no longer exist in PE-UK. I do not think we should merge the model removal until the companion uk-data cleanup lands and verifies the generated SPI/enhanced datasets still load against the new variable set. The formula changes themselves look reasonable; the blocker is the cross-repo dataset contract.

[MaxGhenis]

Clarifying the review body above: the first-party policyengine-uk-data pipeline still writes these removed PE-UK variable names in policyengine_uk_data/datasets/spi.py:

• trading_allowance
• property_allowance

So the blocker is not the formula simplification itself; it is merging the PE-UK variable removal before the companion uk-data cleanup verifies SPI/enhanced datasets no longer emit or rely on those columns.

[MaxGhenis]

Fixed the blocker two ways:

1. Pushed f41d3b7 here so taxable_property_income now uses the concrete property_allowance_deduction variable, matching taxable_self_employment_income's use of trading_allowance_deduction. That keeps pre-net datasets able to override the actual deduction without reintroducing policy-parameter pass-through inputs.
2. Opened companion uk-data PR Use SPI allowance deduction overrides policyengine-uk-data#385, which moves SPI overrides from trading_allowance / property_allowance to trading_allowance_deduction / property_allowance_deduction and adds a regression test.

Local checks here:

• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/income_tax/bases/taxable_property_income.yaml policyengine_uk/tests/policy/baseline/gov/hmrc/income_tax/bases/taxable_self_employment_income.yaml -c policyengine_uk -> 23 passed
• uv run ruff check policyengine_uk/variables/gov/hmrc/income_tax/bases/taxable_property_income.py -> passed

[MaxGhenis]

Reviewed after the companion uk-data fix. This now removes the pass-through allowance variables while keeping the concrete deduction variables as the data override point; taxable_property_income uses property_allowance_deduction, matching the self-employment path. Local targeted YAML tests passed (23 cases), ruff passed, and GitHub CI is green.