Default property_purchased to False to stop phantom stamp duty (#1752)

property_purchased had default_value=True, introduced incidentally in
the 2025 one-variable-per-file refactor (#1139). Because
main_residential_property_purchased is computed as
main_residence_value * property_purchased, a True default charges every
household in a population dataset full SDLT/LBTT/LTT on its entire home
value (~£370bn of phantom stamp duty, 26x real receipts).

This inflated the first income decile's effective tax rate to 251% and
broke the policyengine-uk-data build (test_first_decile_tax_rate_
reasonable) for ~2 weeks, blocking all new UK data releases.

Flip the default to False (fail-safe: a household has not bought all its
property this year). The household calculator and YAML tests set
main_residential_property_purchased directly, so they are unaffected
(all 16 SDLT/LTT tests still pass). Population datasets explicitly set
property_purchased for the ~3.85% of genuine purchasers.

Adds a regression test: a household with main_residence_value but no
purchase now pays £0 stamp duty.

Author: MaxGhenis

changelog.d/fix-property-purchased-default-false.fixed.md

@@ -0,0 +1 @@
+Fix property_purchased default from True to False so households are not charged stamp duty on their entire property wealth unless they explicitly purchased this year. The True default charged every household in population datasets full SDLT/LBTT/LTT on its home value, inflating the first income decile's effective tax rate to 251% and breaking the data pipeline.

policyengine_uk/tests/policy/baseline/gov/hmrc/stamp_duty/property_purchased_default.yaml

@@ -0,0 +1,8 @@
+- name: Household with a home but no purchase pays no stamp duty (property_purchased defaults False)
+  period: 2026
+  input:
+    main_residence_value: 400_000
+  output:
+    property_purchased: false
+    main_residential_property_purchased: 0
+    stamp_duty_land_tax: 0

policyengine_uk/variables/input/consumption/property/property_purchased.py

@@ -7,4 +7,12 @@ class property_purchased(Variable):
     entity = Household
     definition_period = YEAR
     value_type = bool
-    default_value = True
+    # Fail-safe default: a household has NOT bought all its property this year.
+    # main_residential_property_purchased is computed as
+    # main_residence_value * property_purchased, so a True default charges
+    # every household stamp duty on its full property wealth (~£370bn of
+    # phantom SDLT). Population datasets set this explicitly for the small
+    # share of genuine purchasers; the household calculator and tests set
+    # main_residential_property_purchased directly. False is the correct
+    # neutral default for any household where it is not set.
+    default_value = False