Disability benefits: PIP/DLA/AA/Carer's tests and documentation

[vahid-ahmadi]

Addresses #44.

Background

When investigating issue #44, the core disability-benefit modelling was found to already be implemented on main:

• Amount functions (src/variables/benefits.rs): pip_daily_living_amount, pip_mobility_amount, dla_care_amount, dla_mobility_amount, attendance_allowance_amount — each computes weekly rate × 52 from FRS rate-band flags, preserving any FRS-recorded amount.
• Carer's Allowance (calculate_carers_allowance): flat weekly rate × 52 with the net-earnings earnings test (SS (CA) Regs 2002 reg.8).
• Disability premiums (disability_premiums_weekly): DP / EDP / SDP / Carer Premium feeding IS/HB/ESA/JSA applicable amounts.
• Passporting: UC LCWRA element (PIP DL / DLA care mid-high / ESA support proxy), UC carer element, UC disabled child element, and benefit-cap exemption for PIP/DLA/AA/CA recipients.
• Parameters: PipParams, DlaParams, AaParams, DisabilityPremiumParams, IncomeRelatedBenefitParams structs in src/parameters/mod.rs with 2025/26 weekly rates set in parameters/2025_26.yaml (annualised ×52 per codebase convention).
• Disability benefit amounts surface into net income via passthrough_benefits → total_benefits → household net income.

Scope (this PR)

This PR completes the issue by adding the coverage and documentation that were missing:

• Unit tests for the disability/carer amount functions in src/variables/benefits.rs:
  • Enhanced PIP both components → expected annual amount (110.40 + 77.05) × 52
  • Standard PIP daily living only (no mobility)
  • FRS recorded-amount preservation overrides rate-band computation
  • DLA highest-care + higher-mobility for an under-16
  • Higher-rate Attendance Allowance
  • Carer's Allowance earnings-test cliff-edge boundary (full CA just below the £151/wk disregard; withdrawn above it)
• LEGISLATIVE_REFERENCE.md §13A documenting PIP/DLA/AA/Carer's Allowance rates, eligibility, the CA earnings test, and passporting into UC/legacy means-tested benefits.
• Changelog fragment.

Remaining

• Carer's Allowance / LCWRA new-claimant modelling: CA receipt is gated on FRS-reported receipt plus the earnings test rather than computed from caring hours × care-recipient eligibility (the ca_min_hours_caring / ca_care_recipient_min_age parameters are loaded but not yet enforced, since the FRS cannot reliably link carer to care recipient). Similarly the LCWRA flag is a PIP/DLA/ESA proxy rather than a modelled Work Capability Assessment outcome.
• Transitional protection for legacy→UC and DLA→PIP migration is not modelled.

Tests

cargo build and cargo test pass — 196 tests, 0 failures.

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