Route historical under-65 Housing Benefit pensioner allowances

[anth-volk]

Fixes #2230

Implements approved B1 only: the three historical aged_under_65.yaml parameter documents, the applicable-amount consumer, its existing YAML test file and a changelog. Six files, no inputs and no CTR changes. The lower amounts are £151.20/£155.60/£159.35/£163.00 for single/lone-parent families and £230.85/£237.55/£243.25/£248.80 for couples in 2015–18. The existing higher-category correction stays in #2207, which is this PR's base.

Each new document contains a weekly amount and dated GB/NI age limits. A qualifying pension-age-regulation family uses the lower amount only while the eldest claimant/partner is below the category's age limit; age alone does not select the pension-age regulations. Explicit lower-amount reforms, including zero, are retained.

Government citations are embedded at each dated amount: GB 2015, 2016, 2017, 2018, and matching NI 2015, 2016, 2017, 2018 amendments.

GB removes the category on 6 December 2018, under SI 2017/1187 regulations 1 and 7. NI's independently checked 2019 replacement table has no lower category from 1 April 2019. Original NI Orders were used because the consolidated historical table contains stale figures. The annual calculation retains the repository's 30-April sampling: 2018 uses the former category, 2019 no longer does. This does not claim daily age/household-history reconstruction within 2018–19.

Validation: the altered applicable-amount YAML file passed all 13 tests, including seven new tests with 52 household cases across 2015–18, age 64/65, all three family types, both jurisdictions, annual abolition and zero reform. make format and git diff --check passed. Existing core 3.32.13 environment; no full suite, microsimulation or CI polling.

Merge #2207 before this PR. Existing #2092 → #1925 → #1913 still owns current-year values/projections and later cohorts; their rebases must retain this historical selector rather than overwrite it.

axiom: needed

[anth-volk]

Superseded by #2207 at the user’s request. The complete historical under-65 allowance implementation is now included there alongside the higher-category history and corrected regression tests. #2207 references both tracking issues #2206 and #2230; those issues remain open until the combined PR merges. Nothing was merged into main.