Fix UC housing costs element floor

[MaxGhenis]

Summary

• Floor the Universal Credit housing costs element at zero after non-dependent deductions
• Add a YAML regression where non-dependent deductions exceed eligible housing costs

Validation

• uv run --python 3.13 policyengine-core test policyengine_uk/tests/policy/baseline/finance/benefit/family/universal_credit/housing_costs_element/uc_housing_costs_element.yaml -c policyengine_uk
• uv run --python 3.13 ruff format --check policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/uc_housing_costs_element.py
• uv run --python 3.13 ruff check policyengine_uk/variables/gov/dwp/universal_credit/housing_costs_element/uc_housing_costs_element.py
• Axiom local EFRS comparison, 1,000 rows, with this PE worktree plus Axiom award-expression fix: 37,154 compared values, 0 mismatches, 0 oracle divergences

Context

Axiom RuleSpec validation found PE-UK can emit negative uc_housing_costs_element values when non-dependent deductions exceed eligible housing costs. This caused 164 / 37,152 sampled EFRS comparison values to mismatch before this fix.