Bump policyengine-us pin to 1.687.0

[MaxGhenis]

Summary

• Bump the us/dev extras from policyengine-us 1.667.1 to 1.687.0.
• Refresh the bundled US release manifest and lockfile metadata, including policyengine-core 3.26.0.
• Keep the certified US data release at 1.78.2 and update manifest/model tests.

Tests

• .venv/bin/python -m pytest tests/test_release_manifests.py tests/test_models.py tests/test_extra_variables.py -q
• .venv/bin/python -m ruff check tests/test_release_manifests.py tests/test_models.py tests/test_extra_variables.py
• .venv/bin/python import smoke for policyengine 4.3.1, policyengine-us 1.687.0, policyengine-core 3.26.0
• env -u UV_FROZEN uv lock --check
• git diff --check