Apply Housing Benefit main-phase ESA personal allowances

[anth-volk]

Fixes #2216

Adds the approved Housing Benefit main-phase ESA adult allowance exception, using the existing younger/older monetary parameters. The claimant-specific Boolean housing_benefit_has_main_phase_esa delegates legal assessment of GB Schedule 3 paragraph 1A and NI Schedule 4 Part I paragraph 1A. A derived adjustment replaces the lower age-based amount; it is not a second personal allowance. Government citations are included in the source.

Approved plan C4 only: five files and one Boolean input. Reported annual ESA payments do not establish the phase or entitlement condition. No dataset mapping has been verified: omitted/false input retains the age-based assumption and can understate qualifying awards. The variable documentation states this limitation. Pension-age allowances and adult premiums are unchanged; no full ESA entitlement model is added. #1925 continues to own published adult rates.

Validation: 19 new YAML cases passed, including GB/NI, assessment/main phase, under-age single/lone-parent/couple, older adults, missing phase, pension-age exclusion, and a parameter reform reducing the older allowance below the younger amount. make format and git diff --check passed. Existing core 3.32.13 environment; no full suite, population simulation or CI polling.

Merge dependency: independent addition, but preserve the final adjustment when rebasing #1913 or the child/family allowance additions, which share the applicable-amount formula.

axiom: needed