Limit Housing Benefit childcare to qualifying charges and earnings residuals

[anth-volk]

Fixes #2228

Implements approved plan B4 only: five files and one annual GBP input per child for externally pre-assessed qualifying charges. Counts only included children with positive qualifying paid-care costs, reuses WTC's one-/multiple-child caps, and limits the deduction to remaining claimant/partner earnings plus WTC/CTC payments. Generic childcare expenditure is no longer treated as legally qualifying by default.

Government sources are embedded in the formulas and input metadata: GB working-age regulations 27/28, GB pension-age regulations 30/31, NI working-age regulations 25/26, NI pension-age regulations 28/29, and DWP BW2 paragraphs W2.178–W2.182. The residual against WTC/CTC follows DWP's instruction to use any payments, not unrelated income.

The additional earnings disregard remains conditional on net earnings covering the childcare deduction too. Computing the childcare limit before that conditional amount avoids a circular calculation; whenever the additional amount qualifies, the full capped childcare deduction already fits. Standard/special/permitted-work and accommodation amounts reduce available earnings first.

Data limitation: no dataset mapping has been verified. Provider, age, work/leave, incapacity and continuation conditions are assessed outside the model in the approved input. Missing/zero charges give no deduction and can understate awards; zero is not proof of no qualifying expense. No new eligibility flag matrix, household-history model or dataset changes.

Merge order: #1925 supplies fixed £175/£300 caps; this PR does not duplicate that correction. The PR is stacked on #2219, following #2225. Merge #2225 → #2219 → this PR; first-year accommodation timing follows it.

Validation: the altered childcare YAML file passed all 36 cases (GB/NI, working/pension-age regimes, paid-care counts, caps, low/zero earnings, WTC/CTC residuals, special/permitted/accommodation interactions, additional-disregard equality, and cap reform). The initial run failed on an incorrect claimant fixture name; the corrected file passes. make format and git diff --check passed in the existing core 3.32.13 environment. No full suite, microsimulation, documentation build or CI polling.

axiom: needed