Update LCFS, WAS, and ETB prerequisites

[MaxGhenis]

Summary

• Add current private-source release metadata for LCFS 2023-24 (UKDS SN 9468), WAS Round 8 / 2006-2022 (SN 7215), and ETB 1977-2024 (SN 8856).
• Switch private prerequisites to the new expected zip names: lcfs_2023_24.zip, was_2006_22.zip, and etb_1977_24.zip.
• Point LCFS, WAS, ETB, VAT, fuel, and public-services model training at the current tab filenames, with release-specific QRF cache names and metadata checks so stale .pkl files are not reused.
• Update the WAS mapping to Round 8 variables and advance ETB VAT training to year == 2023 for FYE 2024.

Data inputs

The current restricted UKDS TAB zips have been fetched through the UKDS catalogue download endpoint and uploaded to the private HF repo policyengine/policyengine-uk-data-private under the filenames this PR expects:

• lcfs_2023_24.zip: UKDS SN 9468 TAB package 9468tab_A03FF22348E5E7D12FFD971D315D8E54BDC3CE7F6395D0108D85DEEBDF6BE8E3_V1.zip, SHA256 a03ff22348e5e7d12ffd971d315d8e54bdc3ce7f6395d0108d85deebdf6be8e3.
• was_2006_22.zip: UKDS SN 7215 TAB package 7215tab_DF5E8BE49E51AA70F4BF686B98AB44EF11EE1EB260CBAF51308F89DC62449AE1_V1.zip, SHA256 df5e8be49e51aa70f4bf686b98ab44ef11ee1eb260cbaf51308f89dc62449ae1.
• etb_1977_24.zip: UKDS SN 8856 TAB package 8856tab_96FFF4868745A2A9BB1169DCECDAF9958C1179D81F9EF67494C1F29622C7D405_V1.zip, SHA256 96fff4868745a2a9bb1169dcecdaf9958c1179d81f9ef67494c1f29622c7d405.

The downloader now flattens each source package's UKDA-*-tab/tab folder, because the current UKDS TAB zips are nested and the data filenames themselves are not prefixed with the study number.

Refs #411

Tests

• uv run ruff format --check .
• uv run ruff check policyengine_uk_data/datasets/private_releases.py policyengine_uk_data/storage/download_private_prerequisites.py policyengine_uk_data/datasets/imputations/wealth.py policyengine_uk_data/datasets/imputations/consumption.py policyengine_uk_data/datasets/imputations/vat.py policyengine_uk_data/datasets/imputations/services/etb.py policyengine_uk_data/tests/test_frs_prerequisites.py policyengine_uk_data/tests/test_student_loan_balance.py policyengine_uk_data/tests/test_road_fuel_volume_uprating.py policyengine_uk_data/tests/test_vat_parameters.py policyengine_uk_data/tests/test_private_releases.py
• uv run pytest policyengine_uk_data/tests/test_frs_prerequisites.py policyengine_uk_data/tests/test_private_releases.py policyengine_uk_data/tests/test_student_loan_balance.py policyengine_uk_data/tests/test_road_fuel_volume_uprating.py policyengine_uk_data/tests/test_vat_parameters.py -q
• HF round-trip check for lcfs_2023_24.zip, was_2006_22.zip, and etb_1977_24.zip: downloaded from policyengine/policyengine-uk-data-private, verified SHA256, and verified extractor output contains the expected household/person tab files.

[MaxGhenis]

CI status: lint, changelog, and release-manifest checks pass. The Test job fails during make download because HF does not yet have lcfs_2023_24.zip (and the later WAS/ETB zips would be missing too). This matches the draft blocker in the PR body.