Devolved nations: Scottish income tax tests + SCP uprating fix (#46)

[vahid-ahmadi]

Addresses #46

Most of the devolved infrastructure already landed on main (commit 17324e5 added LBTT/LTT; Scottish income tax bands, is_in_scotland dispatch, and Scottish Child Payment were already wired and parameterised). This PR closes the two genuine remaining gaps the issue called out: the missing worked-example/divergence test coverage for Scottish income tax, and a documented data bug in the Scottish Child Payment amount.

Scope (this PR)

• Scottish income tax — worked-example tests. Five new unit tests in src/variables/income_tax.rs pinning HMRC/Revenue-Scotland-style figures for the 2025/26 six-band structure (starter 19% / basic 20% / intermediate 21% / higher 42% / advanced 45% / top 48%):
  • £30k → £3,482.82, £50k → £9,013.80, £75k → £19,513.80, £150k (PA fully tapered) → £59,666.10.
  • An explicit Scotland-vs-rUK divergence test at the same income: a £50k Scottish taxpayer pays ~£1,527.80 more than rUK (42% vs 40% higher rate + lower threshold), while a £15k earner pays less via the 19% starter rate.
• Scottish Child Payment uprating fix. Corrected the stale weekly_amount of £26.70 that was carried across every year: now £27.15 for 2025/26 (SSI 2025/100), £28.20 for 2026/27 (April 2026 uprating), and projected ~CPI uprating for 2027/28–2029/30 (£28.75/£29.34/£29.93), matching the repo's convention for other benefits. The existing SCP test reads from params, so it remains valid. Updated LEGISLATIVE_REFERENCE.md (the "known issues" and Appendix B.10 notes had flagged exactly this bug).

Remaining (out of scope, to document)

• LBTT/LTT additional-dwelling supplement (ADS / higher-rates) — only primary-residence residential bands are modelled.
• Other devolved Scottish benefits — Best Start Grant, Carer's Allowance Supplement, Funeral Support Payment.
• Devolved Council Tax Reduction variants (Scottish/Welsh CTR rules differ from English CTR).
• Scottish Landfill Tax.

Tests

cargo build and cargo test both pass — 195 tests, 0 failures, including all new Scottish income tax and divergence tests.

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