Fix PUF SOI aggregate path fallback

[MaxGhenis]

Summary

• make the PUF uprater load SOI aggregates lazily from storage/calibration_targets/soi_targets.csv
• remove the stale dependency on storage/soi.csv
• drop the now-dead soi.csv private prerequisite download
• add focused unit coverage for the tracked-path and missing-file cases

Validation

• UV_PYTHON=3.14 uv run pytest tests/unit/datasets/test_uprate_puf.py -q
• UV_PYTHON=3.14 uv run ruff check policyengine_us_data/datasets/puf/uprate_puf.py policyengine_us_data/storage/download_private_prerequisites.py tests/unit/datasets/test_uprate_puf.py
• UV_PYTHON=3.14 uv run ruff format --check policyengine_us_data/datasets/puf/uprate_puf.py policyengine_us_data/storage/download_private_prerequisites.py tests/unit/datasets/test_uprate_puf.py
• manual repro: fresh PUF_2021().generate() now gets past the previous NameError: soi is not defined crash