Cache US target reform materialization

[MaxGhenis]

Summary

• add an opt-in --target-materialization-cache-dir for expensive US fiscal target materialization
• cache JCT reform household income-tax vectors by base H5 SHA, target registry version, PE-US version, build commit, period, seed, and reform identity
• report cache hit/miss/write stats in target compilation diagnostics
• extend the JCT materialization regression test to prove cache hits skip reform microsim batches

Validation

• uv run --python 3.13 ruff format tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run --python 3.13 ruff check tools/build_us_fiscal_refresh_release.py packages/populace-build/tests/test_us_fiscal_refresh_builder.py
• uv run --python 3.13 --package populace-build --with pytest python -m pytest packages/populace-build/tests/test_us_fiscal_refresh_builder.py -q
• git diff --check