Document policy start years for AA/CA/CAA/IIDB (#373)

[vahid-ahmadi]

Summary

• Confirm start years for some programs #373 asked to confirm the actual policy start years for Attendance Allowance, Carer's Allowance, Constant Attendance Allowance and Industrial Injuries Disablement Benefit — the parameter trees all start at 2015-04-01 because that's when value tracking began, not when the benefits started.
• Researched the statutory start dates and documented them in the four parameters/gov/dwp/*/README.md files:
  • Attendance Allowance: 1971 (National Insurance Act 1970 + SI 1971/621). Current basis SSCBA 1992 s.64.
  • Carer's Allowance: 1976 (as Invalid Care Allowance under SI 1976/409). Renamed 2003. Current basis SSCBA 1992 s.70.
  • Constant Attendance Allowance: 1948 (NIIA 1946, operative from 5 July 1948). Current basis SSCBA 1992 s.104.
  • Industrial Injuries Disablement Benefit: 1948 (NIIA 1946). Current basis SSCBA 1992 Part V s.103.
• Each README also notes explicitly that the parameter tree only tracks rates from 2015-04-01, so back-cast simulations through the earlier window need rates backfilled from the DWP Abstract of Statistics on Benefit Rates.
• Pure documentation; no parameter values change.

Test plan

• Markdown lints cleanly on all four READMEs (link references).
• No simulation impact since values are unchanged.

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