expected_sdlt adds the property sale rate; expected_lbtt and expected_ltt discount purchases twice #2246

What happens

• expected_sdlt returns stamp_duty_land_tax + property_sale_rate + corporate_sdlt. The sale rate (0.045) is added as pounds, so every household gets £0.045 of phantom SDLT.
• expected_lbtt = property_sale_rate × land_and_buildings_transaction_tax and expected_ltt = property_sale_rate × land_transaction_tax multiply. But LBTT and LTT are already charged only on the dataset's purchase draw (main_residential_property_purchased = main_residence_value × property_purchased, a 3.85% seeded draw in policyengine-uk-data). The 4.5% sale rate therefore discounts them a second time.
• On enhanced_frs_2024_25.h5@1.58.0, FY2025 on main: LBTT is £2.21bn, but expected_lbtt, which feeds gov_tax and household_tax, is £0.10bn. After Stop charging property stocks as purchases when a household buys its home #2241, LBTT on main-residence purchases is £0.37bn, and £0.02bn expected.

Fix

Decide one convention: either the purchase draw is the expectation, so drop property_sale_rate from all three, or the taxes are on full stocks times a sale rate. Do not combine both. property_sale_rate.yaml itself is a 2021 placeholder ("TODO add proper calculations"). Add a property test that expected and realised transaction taxes agree in total under the chosen convention.

Found while fixing #2238 (#2241).