Migrate from policyengine v0.x to v4 (prerequisite for TRACE TRO emission)

[MaxGhenis]

Draft. Requires Modal simulation-worker coordination before merge.

Summary

• Bumps `policyengine` from `>0.12.0,<1` to `>=4.3.1,<5`; `policyengine_us` 1.634.9 → 1.653.3; `policyengine_uk` 2.78.0 → 2.88.0.
• Removes the two pre-v4 imports (`from policyengine.simulation import SimulationOptions` and `from policyengine.utils.data.datasets import get_default_dataset`) in favor of api-local equivalents in a new `policyengine_api.libs.simulation_types` module.
• The new `SimulationOptions` is a Pydantic model matching the Modal worker's existing JSON wire contract (country, scope, reform, baseline, time_period, include_cliffs, region, data, model_version, data_version). Owning the type api-side decouples the api from pe.py's internal class layout.
• The new `get_default_dataset` returns the default HuggingFace-hosted dataset URI for a (country, region) pair. State / district regions fall back to the national default, matching prior behavior.
• 9 smoke tests for the new module. Tests pass in isolation; the full api pytest suite requires CloudSQL credentials to import, so end-to-end integration verification lands in CI.

Why this is draft

The Modal simulation worker is pinned to its own pe.py version (independent of this api). Bumping the api's pe.py pin without coordinating with Modal could produce JSON payloads that the worker does not decode correctly. Whoever merges this should:

1. Confirm the Modal worker is either on a pe.py version that accepts the api-local `SimulationOptions` JSON shape, or bump the worker in the same change window.
2. Verify the api-pytest suite runs green in CI (couldn't run locally because tests require `POLICYENGINE_DB_PASSWORD`).
3. Test a reform economy-wide simulation end-to-end through a staging deploy before promoting.

Why this unblocks a lot

Once this merges, #3485 (webapp TRACE TRO emission) becomes a straightforward feature PR:

```python
from policyengine.provenance.trace import (
build_trace_tro_from_release_bundle,
build_simulation_trace_tro,
serialize_trace_tro,
)

... on every simulation completion, build and persist a TRO.

```

Those imports only exist on the v4 line.

Test plan

• New module smoke-tests pass in isolation
• `uv run python -c "from policyengine.provenance.trace import build_trace_tro_from_release_bundle"` succeeds against the new dep pins
• `uv lock` resolves cleanly
• CI green (needs `POLICYENGINE_DB_PASSWORD` secrets)
• Modal-worker side confirmed compatible
• Staging deploy smoke test against a real economy-wide reform

Fixes #3486. Unblocks #3485.

🤖 Generated with Claude Code