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

changelog.d/373.md

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+- Document the actual policy start years for Attendance Allowance (1971), Carer's Allowance (1976, as Invalid Care Allowance), Constant Attendance Allowance (1948) and Industrial Injuries Disablement Benefit (1948) in their `parameters/gov/dwp/*/README.md` files, with statutory references and an explicit note that the parameter tree only tracks rates from 2015-04-01 onwards.

policyengine_uk/parameters/gov/dwp/IIDB/README.md

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 # Industrial Injuries Disablement Benefit
+
+**Policy start year**: 1948. Originally **Industrial Injuries
+Disablement Pension** under the [National Insurance (Industrial
+Injuries) Act 1946][niia-1946], operative from 5 July 1948. Renamed
+*Disablement Benefit* in 1986 and folded into the broader Industrial
+Injuries Scheme.
+
+PolicyEngine UK populates `maximum.yaml` from 2015-04-01 onwards. The
+benefit is paid in 10 percent increments of the maximum rate
+(`gov.dwp.IIDB.maximum`) reflecting the assessed disablement
+percentage; the model treats the reported amount as derived from
+`iidb_reported` rather than re-derived from a percentage. For periods
+before 2015 the parameter resolver returns the 2015 value —
+`Microsimulation` queries on those years would compute against a value
+the policy didn't actually take.
+
+Current statutory basis: [Social Security Contributions and Benefits
+Act 1992 Part V][sscba-part-v] (in particular s. 103) and the [Social
+Security (Industrial Injuries) (Prescribed Diseases) Regulations
+1985][prescribed-diseases-1985] (SI 1985/967).
+
+[niia-1946]: https://www.legislation.gov.uk/ukpga/1946/62/contents
+[sscba-part-v]: https://www.legislation.gov.uk/ukpga/1992/4/part/V
+[prescribed-diseases-1985]: https://www.legislation.gov.uk/uksi/1985/967

policyengine_uk/parameters/gov/dwp/attendance_allowance/README.md

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 # Attendance Allowance
+
+**Policy start year**: 1971. Originally introduced by the
+[National Insurance (Old Persons' and Widows' Pensions and Attendance
+Allowance) Act 1970][nipowpaa-1970] and made operational by the
+[National Insurance (Attendance Allowance) Regulations 1971][naar-1971].
+
+PolicyEngine UK populates `higher.yaml` and `lower.yaml` from
+2015-04-01 onwards (the first year tracked in the parameter tree). For
+periods before 2015 the parameter resolver returns the 2015 value —
+`Microsimulation` queries on those years would compute against a value
+the policy didn't actually take. Back-cast simulations needing the
+1971–2014 window should backfill rates from the DWP *Abstract of
+Statistics on Benefit Rates*.
+
+Current statutory basis: [Social Security Contributions and Benefits
+Act 1992 s. 64][sscba-64] and the [Social Security (Attendance
+Allowance) Regulations 1991][saa-1991] (SI 1991/2740).
+
+[nipowpaa-1970]: https://www.legislation.gov.uk/ukpga/1970/51/contents
+[naar-1971]: https://www.legislation.gov.uk/uksi/1971/621
+[sscba-64]: https://www.legislation.gov.uk/ukpga/1992/4/section/64
+[saa-1991]: https://www.legislation.gov.uk/uksi/1991/2740

policyengine_uk/parameters/gov/dwp/carers_allowance/README.md

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 # Carer's Allowance
+
+**Policy start year**: 1976. Introduced as **Invalid Care Allowance**
+under [The Social Security (Invalid Care Allowance) Regulations 1976][icaa-regs]
+(SI 1976/409), made under section 37 of the [Social Security Act 1975][ssa-1975].
+Renamed **Carer's Allowance** in 2003.
+
+PolicyEngine UK populates `rate.yaml` from 2015-04-01 onwards (the first
+year tracked in the parameter tree). For periods before 2015 the
+parameter resolver returns the 2015 value — `Microsimulation` queries
+on those years would compute against a value the policy didn't actually
+take. If back-cast simulations through the 1976–2014 window are needed,
+the missing rates should be backfilled from the DWP *Abstract of
+Statistics on Benefit Rates*.
+
+Current statutory basis: [Social Security Contributions and Benefits
+Act 1992 s. 70][sscba-70].
+
+[icaa-regs]: https://www.legislation.gov.uk/uksi/1976/409
+[ssa-1975]: https://www.legislation.gov.uk/ukpga/1975/14/contents
+[sscba-70]: https://www.legislation.gov.uk/ukpga/1992/4/section/70

policyengine_uk/parameters/gov/dwp/constant_attendance_allowance/README.md

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 # Constant Attendance Allowance
+
+**Policy start year**: 1948. Introduced under the
+[National Insurance (Industrial Injuries) Act 1946][niia-1946] as one
+of the supplements to the Industrial Injuries Disablement benefit
+package, payable from 5 July 1948 (the appointed day for the post-war
+social security reforms).
+
+PolicyEngine UK populates the per-day-rate parameters
+(`full_day_rate`, `intermediate_rate`, `part_day_rate`,
+`exceptional_rate`) from 2015-04-01 onwards. For periods before 2015
+the parameter resolver returns the 2015 value — `Microsimulation`
+queries on those years would compute against a value the policy didn't
+actually take.
+
+Current statutory basis: [Social Security Contributions and Benefits
+Act 1992 s. 104][sscba-104] and the [Social Security (General Benefit)
+Regulations 1982][gbr-1982] (SI 1982/1408).
+
+[niia-1946]: https://www.legislation.gov.uk/ukpga/1946/62/contents
+[sscba-104]: https://www.legislation.gov.uk/ukpga/1992/4/section/104
+[gbr-1982]: https://www.legislation.gov.uk/uksi/1982/1408