fix(scotland): add 2026-27 higher rate threshold freeze to baseline (#1486)

* fix(scotland): add 2026-27 higher rate threshold freeze to baseline

Scottish Budget 2025-26 announced freezing higher, advanced, and top
rate thresholds through 2026-27. This updates the baseline parameters
to include these frozen values for 2026-04-06.

Without this fix, PolicyEngine's baseline incorrectly assumes CPI
growth for 2026, leading to incorrect policy costings when comparing
to SFC (Scottish Fiscal Commission) estimates.

Thresholds frozen at:
- Higher rate: £31,092
- Advanced rate: £62,430
- Top rate: £125,140

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

* Add changelog entry and apply Black formatting

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
Co-authored-by: Vahid Ahmadi <va.vahidahmadi@gmail.com>

Author: 3 people

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+      - Add 2026-27 Scottish income tax threshold freeze to baseline (higher, advanced, and top rates) per Scottish Budget 2025-26.

policyengine_uk/parameters/gov/hmrc/income_tax/rates/scotland/rates.yaml

@@ -45,6 +45,8 @@ brackets:
         2023-04-06: 31_092
         2024-04-06: 31_092
         2025-04-06: 31_092
+        # Scottish Budget 2025-26 announced freeze through 2026-27
+        2026-04-06: 31_092
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -57,6 +59,8 @@ brackets:
         2023-04-06: 125_140
         2024-04-06: 62_430
         2025-04-06: 62_430
+        # Scottish Budget 2025-26 announced freeze through 2026-27
+        2026-04-06: 62_430
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -69,6 +73,8 @@ brackets:
         2017-04-06: null
         2024-04-06: 125_140
         2025-04-06: 125_140
+        # Scottish Budget 2025-26 announced freeze through 2026-27
+        2026-04-06: 125_140
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -81,3 +87,5 @@ metadata:
   reference:
     - title: GOV.UK
       href: https://www.gov.uk/government/publications/rates-and-allowances-income-tax/income-tax-rates-and-allowances-current-and-past
+    - title: Scottish Budget 2025-26 - Higher rate threshold freeze
+      href: https://www.gov.scot/publications/scottish-budget-2025-26/