UC higher disabled child addition: add the blindness limb and stop keying it on the tax credit severe flag #1970

uc_individual_severely_disabled_child_element has defined_for = "is_severely_disabled_for_benefits". That flag is the Child Tax Credit / Working Tax Credit severe disability condition: DLA care highest rate, PIP daily living enhanced rate, and (after #1946) higher-rate Attendance Allowance or Armed Forces Independence Payment.

UC Regs 2013 reg 24(2)(b) (current text, read 2026-09-30) gives the higher rate where the child or qualifying young person is:

• (i) entitled to the DLA care component at the highest rate, the Scottish adult DLA or child disability payment care component at the highest rate, the ADP daily living component at the enhanced rate, or the PIP daily living component at the enhanced rate; or
• (ii) blind, meaning certified as severely sight impaired or blind by a consultant ophthalmologist (reg 2).

Reg 24(2)(b) has no AFIP or Attendance Allowance limb.

The model therefore misses blind children: is_blind exists as an input. Separately, children cannot receive AFIP or AA, so those limbs are inert in practice.

Proposed fix: give the UC higher rate its own explicitly named condition variable that includes the blind limb, keep is_severely_disabled_for_benefits for CTC/WTC, and add YAML tests (a blind child with no DLA/PIP gets the higher rate) plus a property test (the higher and lower rates are exclusive). Run the Enhanced FRS impact with real runs.

Found while reviewing #1946.

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