Clarify post-CTR council tax semantics

[MaxGhenis]

Summary

This is the first contingent post-CTR pipeline PR. It is intentionally scoped to semantics and tests so it can sit behind the UK-wide CTR implementation without changing production net-income behavior.

Changes:

• Clarifies that council_tax is the dataset-supplied gross annual Council Tax liability, not a value recomputed from local authority schedules.
• Renames/document council_tax_less_benefit as Council Tax after Council Tax Reduction and makes the zero floor explicit.
• Adds a regression test that net Council Tax after CTR cannot go below zero.
• Adds docs/council_tax_ctr_pipeline.md with the proposed draft PR sequence for structural CT and data-pipeline calibration work.

Dependency

Blocked on #1534 completing UK-wide CTR coverage. Until then, downstream changes should remain compare-only and should not switch household net income or production calibration outputs onto modelled CTR.

Validation

• uv run policyengine-core test policyengine_uk/tests/policy/baseline/household/consumption/council_tax_less_benefit.yaml -c policyengine_uk

Note: running the global policyengine-core directly failed because it used a Python environment where this checkout was not importable; the repo environment command above passed.