Add US long-run bundle manifest

[MaxGhenis]

Summary

Adds the CRFB US long-run managed dataset bundle to policyengine.py so downstream runs can resolve published 2026-2100 H5s through the normal managed dataset manifest instead of local policyengine-us / policyengine-us-data paths.

Key changes:

• Updates the US release manifest to policyengine-us==1.691.12 and the crfb-longrun-20260517 policyengine-us-data release manifest.
• Adds long-term dataset entries with H5 and metadata hashes.
• Adds manifest resolution and US dataset helpers/tests for long-term managed datasets.
• Pins the US extra to policyengine_core==3.26.1 with policyengine-us==1.691.12, matching the model package used to build the released long-run H5s.

Validation

• uv run --with ruff ruff check src/policyengine/tax_benefit_models/us/datasets.py src/policyengine/provenance/manifest.py tests/test_us_long_term_datasets.py tests/test_release_manifests.py tests/test_models.py
• uv run pytest tests/test_us_long_term_datasets.py tests/test_release_manifests.py tests/test_models.py -q (78 passed, 1 warning)

Notes

The long-run H5s are published at Hugging Face tag crfb-longrun-20260517; downstream CRFB sentinel runs have resolved and scored 2100 through this manifest path using the local worktree.