Add Housing Benefit permitted-work earnings disregard

[anth-volk]

Fixes #2218

Adds the approved pre-assessed annual Housing Benefit permitted-work disregard. The supplied amount replaces the ordinary earnings disregard, retains a higher lone-parent amount, is capped at net earnings, and leaves the additional and accommodation disregards separately conditional. The statutory earnings, work and partner-allocation limits are assessed externally, not inferred from gross work income.

Government sources embedded in code: GB working-age Schedule 4 paragraph 10A, GB pension-age paragraph 5A, and the matching NI working-age and pension-age provisions.

Approved plan C6 only: four files, one GBP input. No dataset mapping has been verified. Missing/zero input preserves the ordinary-disregard approximation and can understate qualifying awards; public variable documentation explains this. No new ESA or permitted-work eligibility engine.

Validation: 32 new YAML cases passed in the existing core 3.32.13 environment. Covers GB/NI and both regulation sets, replacement, lone-parent precedence, low earnings, additional-disregard equality and failure, accommodation, parameter reform, passport, and replacement of the special standard amount. The six special-disregard tests also pass on the composed branch. make format and git diff --check passed. No full suite, population simulation or CI polling.

Merge dependency: stacked on #2225 (special standard disregard); merge #2225 first, then this PR. The childcare and first-year accommodation changes follow this replacement.

axiom: needed