Add PolicyEngine US simulation target compiler

[MaxGhenis]

Summary

• bump Microplex to the fresh-engine commit with SimulationTargetCompiler support
• tag PolicyEngine US canonical target rows that need PE materialization or rerandomized takeup
• add PolicyEngineUSSimulationTargetCompiler to force calculated-output materialization, apply modifier handlers, and emit sparse household-weight constraints
• pin CI editable Microplex checkouts to the same core commit as pyproject so workflow tests do not override the dependency with stale core main

Validation

• env -u UV_FROZEN uv run --extra dev --extra policyengine pytest tests/test_target_adapters.py tests/policyengine/test_us.py -q
• env -u UV_FROZEN uv run --extra dev --extra policyengine pytest tests/policyengine/test_comparison.py -q
• env -u UV_FROZEN uv run --extra dev ruff check src/microplex_us/policyengine/us.py src/microplex_us/targets/adapters.py src/microplex_us/policyengine/init.py src/microplex_us/init.py tests/test_target_adapters.py tests/policyengine/test_us.py
• git diff --check