Default property_purchased to False to stop phantom stamp duty

[MaxGhenis]

Problem

property_purchased had default_value = True (introduced incidentally in the 2025 one-variable-per-file refactor, #1139). Because main_residential_property_purchased = 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, ~26× real receipts.

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

Fix

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 YAML tests still pass.
• Population datasets explicitly set property_purchased for the ~3.85% of genuine purchasers (see the paired policyengine-uk-data determinism PR).

Test plan

• All SDLT + LTT baseline YAML tests pass (16 cases).
• New regression test: a household with main_residence_value but no purchase now computes stamp_duty_land_tax: 0 (was full SDLT).
• Verified end-to-end: £400k home, no explicit purchase → property_purchased=False, main_residential_property_purchased=£0, stamp_duty_land_tax=£0.