Integrate property_income_tax into total income_tax (#1408)

* Integrate property_income_tax into total income_tax

This fix ensures that property income is taxed at the property-specific
rates (22%, 42%, 47% from April 2027) rather than the standard earned
income rates.

Changes:
- Add taxable_property_income to earned_taxable_income_exclusions
  (prevents double-taxation of property income)
- Add property_income_tax to income_tax_pre_charges
- Add property_income_tax to income_tax_additions
- Add integration tests verifying property_income_tax flows into income_tax

This implements the November 2025 Autumn Budget property income tax
increases correctly, allowing microsimulation to show the revenue
impact of the +2pp rate increase.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* Update expected reform impacts after property tax integration

The integration of property_income_tax into total income_tax slightly
shifts the baseline, which changes the expected fiscal impacts of
other reforms by small amounts.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

---------

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

Author: MaxGhenis

policyengine_uk/parameters/gov/hmrc/income_tax/earned_taxable_income_exclusions.yaml

@@ -3,9 +3,10 @@ values:
   0000-01-01:
     - taxable_savings_interest_income
     - taxable_dividend_income
+    - taxable_property_income
     - received_allowances_earned_income
     - marriage_allowance
-    
+
 metadata:
   unit: list
   label: Earned taxable income exclusions

policyengine_uk/parameters/gov/hmrc/income_tax/income_tax_additions.yaml

@@ -7,5 +7,6 @@ values:
   - earned_income_tax
   - savings_income_tax
   - dividend_income_tax
+  - property_income_tax
   - CB_HITC
   - personal_pension_contributions_tax

policyengine_uk/tests/microsimulation/reforms_config.yaml

@@ -4,7 +4,7 @@ reforms:
   parameters:
     gov.hmrc.income_tax.rates.uk[0].rate: 0.21
 - name: Raise higher rate by 1pp
-  expected_impact: 5.3
+  expected_impact: 5.0
   parameters:
     gov.hmrc.income_tax.rates.uk[1].rate: 0.42
 - name: Raise personal allowance by ~800GBP/year
@@ -16,18 +16,18 @@ reforms:
   parameters:
     gov.hmrc.child_benefit.amount.additional: 25
 - name: Reduce Universal Credit taper rate to 20%
-  expected_impact: -30.2
+  expected_impact: -31.0
   parameters:
     gov.dwp.universal_credit.means_test.reduction_rate: 0.2
 - name: Raise Class 1 main employee NICs rate to 10%
-  expected_impact: 13.0
+  expected_impact: 13.2
   parameters:
     gov.hmrc.national_insurance.class_1.rates.employee.main: 0.1
 - name: Raise VAT standard rate by 2pp
   expected_impact: 18.8
   parameters:
     gov.hmrc.vat.standard_rate: 0.22
 - name: Raise additional rate by 3pp
-  expected_impact: 4.6
+  expected_impact: 4.5
   parameters:
     gov.hmrc.income_tax.rates.uk[2].rate: 0.48

policyengine_uk/tests/policy/reforms/nov_2025_budget/income_source_tax_rates.yaml

@@ -182,3 +182,54 @@
   output:
     # £1000 at NEW rate 47%
     property_income_tax: 1000 * 0.47
+
+# =============================================================================
+# INTEGRATION TESTS - verify income_tax includes source-specific taxes
+# These tests verify that property_income_tax flows into total income_tax
+# =============================================================================
+
+- name: Property tax flows into income_tax - basic rate - 2028
+  period: 2028
+  absolute_error_margin: 1
+  input:
+    # Person with just personal allowance worth of employment income
+    employment_income: 12570
+    property_income: 2000
+  output:
+    # Property allowance is £1000, so £1000 taxable at 22%
+    property_income_tax: 220
+    # Total income_tax should include property_income_tax
+    # Employment: £0 (covered by PA), Property: £220
+    income_tax: 220
+
+- name: Property tax flows into income_tax - higher rate - 2028
+  period: 2028
+  absolute_error_margin: 1
+  input:
+    employment_income: 60000
+    property_income: 2000
+  output:
+    # £1000 at 42%
+    property_income_tax: 420
+    # income_tax = earned_income_tax + savings_income_tax + dividend_income_tax + property_income_tax
+    # Earned income tax on £60000: PA=£12570, taxable=£47430
+    # Basic rate (20%): £37700 at 20% = £7540
+    # Higher rate (40%): £9730 at 40% = £3892
+    # Total earned: £11432
+    # Property: £420
+    # Total: £11852
+    income_tax: 11852
+
+- name: Savings tax flows into income_tax - higher rate - 2028
+  period: 2028
+  absolute_error_margin: 1
+  input:
+    employment_income: 60000
+    savings_interest_income: 2000
+  output:
+    # £1500 at 42% (after £500 savings allowance)
+    savings_income_tax: 630
+    # Earned: £11432 (same as above)
+    # Savings: £630
+    # Total: £12062
+    income_tax: 12062

policyengine_uk/variables/gov/hmrc/income_tax/income_tax_pre_charges.py

@@ -16,4 +16,5 @@ class income_tax_pre_charges(Variable):
         "earned_income_tax",
         "savings_income_tax",
         "dividend_income_tax",
+        "property_income_tax",
     ]