Add Housing Benefit special £20 earnings disregard

[anth-volk]

Fixes #2224

Adds the approved £20 Housing Benefit special standard earnings disregard. It replaces £5/£10 rather than stacking with them, preserves lone-parent precedence, caps the deduction at net earnings and participates in the additional-disregard threshold.

Automatic routes distinguish the statutory regulation set: working-age disability/severe premiums; pension-age qualifying benefit receipt, blindness or carer premium, restricted to claimant/partner. Other full-amount cases can use the single pre-assessed Boolean. Existing premium qualification approximations are inherited, not audited or expanded here.

Important boundary: working-age carer/occupation provisions can restrict the deduction to a particular person's/job's earnings. A supplied Boolean must establish that the full amount is legally available across the unit's earnings. Category membership alone does not suffice. Partial allocations below £20 and missing ESA phase/continuity facts are not reconstructed; the false fallback can omit qualifying awards. No verified dataset mapping for residual pre-assessment.

Government references are embedded in the monetary parameter and all formulas: GB working-age Schedule 4, GB pension-age Schedule 4, NI working-age Schedule 5, NI pension-age Schedule 5. The £20 amount is not CPI-uprated.

Approved plan C5 only: seven files, at most one Boolean input; no occupation/history flag matrix.

Validation: the special-disregard file passed six YAML tests, containing 31 individual household cases for GB/NI, automatic routes and negative controls, replacement, couple total, lone-parent precedence, low earnings, additional-disregard equality, passport and a zero-value reform. make format and git diff --check passed. Existing core 3.32.13 environment; no full suite, population simulation or CI polling.

Updated the existing disabled-worker regression to assert the implemented special amount: (£20 + £17.10) × 52 = £1,929.20, rather than the obsolete (£5 + £17.10) × 52 expectation. Removed its known-limitation name/comments and retained primary-source citations. No formula, parameter or input changes in this follow-up. The complete altered disregard YAML file passed 30 tests, and the special-disregard YAML file passed 6 tests, sequentially: 36 passed. Formatting/lint and git diff --check passed. No full suite or CI polling; hosted CI on the new commit is not confirmed. Dependent PRs #2219/#2229/#2234 were not rewritten by this follow-up and must retain this regression correction when rebased.

Merge dependency: merge before the permitted-work and childcare/accommodation composition, preserving each later replacement and calculation.

axiom: needed

[anth-volk]

Corrected the existing disabled-worker test to expect (£20 + £17.10) × 52 = £1,929.20 and removed its obsolete known-limitation description. No formula, parameter or input changes. The altered earnings-disregard YAML file passed 30 tests; the special-disregard file passed 6 tests, sequentially. Formatting/lint and git diff --check passed. No full suite or CI polling. Dependent branches were not rewritten; preserve this correction when rebasing them.