Improve Tax-Free Childcare income test

[MaxGhenis]

Summary

• Add a Tax-Free Childcare declaration-period weeks parameter.
• Add tax_free_childcare_expected_declaration_period_income, defaulting annual countable income into the declaration period instead of hard-coding annual income divided by four inside the eligibility formula.
• Add self-employment start-up period relief for the minimum income threshold.
• Match legislative boundary wording by using >= for the minimum income threshold and <= for the £100k adjusted-net-income limit.

Closes #1043.
Refs #1047.

Notes

• This keeps the model annual, but makes the declaration-period expected income explicit and overrideable. A full multi-quarter time-axis implementation remains a larger Tax-Free Childcare: Implement quarterly reassessment #1047-style model architecture question.

Tests

• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/gov/hmrc/tax_free_childcare -c policyengine_uk
• uv run --python 3.13 ruff check policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_expected_declaration_period_income.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_self_employment_start_up_period.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_meets_income_requirements.py
• uv run --python 3.13 ruff format --check policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_expected_declaration_period_income.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_self_employment_start_up_period.py policyengine_uk/variables/gov/hmrc/tax_free_childcare/conditions/tax_free_childcare_meets_income_requirements.py