Support the Housing Benefit residential capital exception

[anth-volk]

Fixes #2214

Added the approved working-age residential capital tariff exception: a £10,000 threshold and one Boolean benefit-unit input, housing_benefit_residential_capital_exception. The existing tariff formula selects it only under the working-age regulations. Pension-age thresholds, Guarantee Credit passports and Savings Credit-only handling remain unchanged.

The input explicitly delegates assessment of GB regulation 52(4)–(5), (8)–(9) or NI regulation 49(3)–(4), (7)–(8). It is not inferred from in_specified_or_temporary_accommodation, which denotes a different legal category. Government references are embedded in the parameter and both variables.

Approved plan B3 only: five files, one Boolean input. No dataset mapping has been verified. Omitted/false input preserves the ordinary £6,000 assumption and may understate awards for qualifying residents; this limitation is in the variable's public documentation. Capital ownership/valuation, upper-limit behavior and historical residential categorisation are not expanded. Related #1758 remains open because it also covers other programs and the exposed tariff upper-limit question. Existing #1969/#1974/#2131/#2201 are not reimplemented.

Validation: seven new YAML cases passed, including every threshold/partial-step boundary in GB/NI, omitted/default status, an accommodation-flag negative control, pension-age tariff and both PC passports. An initial fixture attempt failed because mixed scalar/array inputs had inconsistent entity counts; the corrected uniform arrays pass. make format and git diff --check passed. No full suite, population simulation or CI polling; existing core 3.32.13 environment.

Merge dependency: independent of the valuation changes; retain this threshold selection when composing with them. Dataset support remains separate.

axiom: needed