Add national council tax reduction core

[MaxGhenis]

Summary

• add a main-based national CTR core for Scotland, Wales, and English pensioner households
• preserve reported CTB/CTR for unsupported English working-age local schemes
• expose council_tax_reduction, support detection, simulated benunit CTR, and net council tax flooring
• add national CTR parameters with primary legislation references

This is a focused extraction from the larger draft CTR framework branch (#1534), leaving local working-age authority schemes for later auditable batches.

Tests

• uv run policyengine-core test policyengine_uk/tests/policy/baseline/gov/local_authorities/council_tax_reduction/council_tax_reduction.yaml policyengine_uk/tests/policy/baseline/consumption/council_tax_less_benefit.yaml -c policyengine_uk
• uv run policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
• uv run --with pytest --with pytest-cov python -m pytest policyengine_uk/tests/ --cov=policyengine_uk --cov-report=xml --maxfail=1 -v
• uv run ruff check policyengine_uk/variables/gov/local_authorities/council_tax_reduction policyengine_uk/variables/gov/dwp/council_tax_benefit.py policyengine_uk/variables/household/consumption/council_tax_less_benefit.py policyengine_uk/variables/input/consumption/property/council_tax.py
• git diff --check