Exclude disregarded income from Housing Benefit assessment

[anth-volk]

Fixes #2210

Removed Child Benefit, Sure Start Maternity Grants and dependent children's own income from the ordinary Housing Benefit means test. The personal-income and tax/pension masks now include only claimant and partner. Existing Guarantee Credit/legacy-benefit/UC passports and the separate Savings Credit-only calculation are unchanged.

Embedded government citations cover GB regulation 25(3), Schedule 5 paragraphs 31 and 65, the pension-age regulations, and the separately verified NI regulation 22 and Schedule 6 paragraphs 32/64. NI numbering differs from GB and was not copied by assumption.

Approved plan A4 only: three files, no inputs. #2027 still owns claimant/partner attribution of legacy awards and passports; #1995 still owns capital-derived income. Both touch the same formula and must retain these exclusions when rebasing. This is not a complete rewrite of income assessment.

Validation: all 13 tests in the altered income YAML file passed: the eight new cases plus the five existing cases, with the old Child Benefit inclusion expectation corrected to its statutory exclusion. The new cases were moved into the existing test file, so this PR still changes only three files. make format and git diff --check passed. Existing core 3.32.13 environment with isolated worktree imports, not a fresh lock sync. No full suite, microsimulation or CI polling.

The same source file now exposes its unchanged pre-deduction income calculation as a pure helper. The separately approved accommodation timing PR can reuse it before applying each segment's income floor, without duplicating source lists or modifying stored variable inputs. This adds no further policy or files. #2225 is now stacked on this PR, followed by #2219 and #2229.

Merge dependency: independent correction, but rebase after #2027 and #1995 to reconcile their shared-file changes.

axiom: needed