Update FRS ingestion for 2024-25

[MaxGhenis]

Summary

• Centralize the current FRS release metadata for UKDS SN 9563 / FRS 2024-25.
• Point the build, test fixtures, manifest defaults, and upload/download helpers at frs_2024_25 and enhanced_frs_2024_25.
• Flatten the UK Data Service nested TAB zip layout during private prerequisite extraction, with tests for the 2024-25 package shape.
• Advance the eFRS base year to 2024 so dataset periods, donor-year alignment, local-area calibration keys, and weight-loading defaults move together.

Closes #409.

Validation

• uv run pytest policyengine_uk_data/tests/test_frs_prerequisites.py policyengine_uk_data/tests/test_release_manifest.py policyengine_uk_data/tests/test_load_weights.py policyengine_uk_data/tests/test_calibrate_save.py -q
• uv run ruff check ... on touched Python files
• uv run ruff format --check ... on touched Python files
• git diff --check

Notes

• This does not upload the restricted raw FRS zip or build/publish new H5 artifacts. frs_2024_25.zip still needs to be mirrored privately before download_prerequisites() can fetch it in a normal data build.
• 2025-only target sources are no longer implicitly used for a 2024-base eFRS; those should be explicitly backcast or aged if we want them in the 2024 calibration surface.