Fix Scottish top rate threshold to use correct above-PA value (#1488)

* Fix Scottish top rate threshold to use correct above-PA value

The Scottish top rate (48%) threshold was incorrectly stored as £125,140
(the total income threshold) instead of £112,570 (the amount above
personal allowance).

PolicyEngine stores income tax thresholds as amounts ABOVE the personal
allowance (£12,570), not as total income. The correct calculation is:
- Total income threshold: £125,140
- Above PA: £125,140 - £12,570 = £112,570

This bug caused the top rate to effectively start at £137,710 total
income instead of £125,140, significantly underestimating the number
of top rate taxpayers and the revenue from threshold freezes.

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

* Add changelog entry

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

* Add SFC reference, unit tests, and fix changelog formatting

- Add Scottish Fiscal Commission January 2026 reference to rates.yaml
- Add two unit tests for Scottish top rate threshold verification
- Remove currency symbols from changelog entry for consistency

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

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: vahid-ahmadi

changelog_entry.yaml

@@ -0,0 +1,7 @@
+- bump: patch
+  changes:
+    fixed:
+    - Scottish top rate (48%) threshold corrected from 125,140 to 112,570 (above
+      personal allowance). The threshold was incorrectly stored as the total income
+      value instead of the amount above PA, causing the top rate to effectively start
+      at 137,710 instead of 125,140.

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

@@ -71,10 +71,12 @@ brackets:
   - threshold:
       values:
         2017-04-06: null
-        2024-04-06: 125_140
-        2025-04-06: 125_140
+        # Top rate threshold: £125,140 total income = £112,570 above PA
+        # (£125,140 - £12,570 personal allowance)
+        2024-04-06: 112_570
+        2025-04-06: 112_570
         # Scottish Budget 2025-26 announced freeze through 2026-27
-        2026-04-06: 125_140
+        2026-04-06: 112_570
       metadata:
         uprating: gov.economic_assumptions.indices.obr.consumer_price_index
     rate:
@@ -89,3 +91,5 @@ metadata:
       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/
+    - title: Scottish Fiscal Commission - Scotland's Economic and Fiscal Forecasts January 2026
+      href: https://fiscalcommission.scot/wp-content/uploads/2026/01/Scotlands-Economic-and-Fiscal-Forecasts-January-2026-revised-13-01-2026.pdf

policyengine_uk/tests/policy/baseline/finance/tax/income_tax/scottish_rates.yaml

@@ -6,3 +6,29 @@
     region: SCOTLAND
   output:
     tax_band: STARTER
+
+- name: Scottish top rate applies at 125140 total income
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    # Top rate (48%) starts at 125,140 total income (112,570 above PA)
+    # At this income, PA is fully tapered, so taxable income = 125,140
+    # This exceeds the top rate threshold of 112,570
+    adjusted_net_income: 125140
+    region: SCOTLAND
+  output:
+    # TaxBand enum uses ADDITIONAL for both Advanced (45%) and Top (48%) rates
+    tax_band: ADDITIONAL
+
+- name: Scottish income above advanced threshold
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    # 100,000 income is above advanced rate threshold (62,430 above PA)
+    # but below top rate threshold (112,570 above PA)
+    # PA at 100,000 is 12,570 (not yet tapered), taxable = 87,430
+    adjusted_net_income: 100000
+    region: SCOTLAND
+  output:
+    # TaxBand enum uses ADDITIONAL for both Advanced (45%) and Top (48%) rates
+    tax_band: ADDITIONAL