Proposal: deterministic PolicyEngine YAML test -> portable fixture converter

[edithatogo]

I have a small deterministic converter prototype that may be useful for PolicyEngine's existing validation and cross-model comparison work: it converts supported PolicyEngine YAML tests into an intermediate fixture JSON format and back to canonical PolicyEngine YAML data.

Current prototype supports

• PolicyEngine YAML test → intermediate fixture JSON → PolicyEngine YAML test round-trips for a documented subset
• Scalar and nested entity input/output maps
• Absolute error margins
• Optional crosswalk remapping for cross-engine conversion when every variable has an explicit mapping
• Loud rejection for unsupported constructs (reforms, extensions, relative error margins, dotted input keys treated as inline parameter reforms, lists, nulls, expression strings)

I verified the current PolicyEngine YAML loader from policyengine-core commit f761573c2a13adecc3826be04af1980d13657e1d (policyengine_core/tools/test_runner.py) before defining the subset.

Corpus evidence

• Source corpus: 10 YAML files from PolicyEngine/policyengine-us commit fc64cef64ab55c3c48309c7fb304c35e5f3c9184
• Converted files: 10/10
• Rejected files: 0/10 in the selected v0.1 subset corpus
• Generated fixture JSON is validated with a JSON Schema validator in CI

This is intended as a migration/cross-validation utility, not a replacement for PolicyEngine's native test format. The converter refuses ambiguous or engine-specific constructs rather than silently dropping them.

Questions

Would PolicyEngine maintainers be open to this living as:

1. an external utility with docs/examples that reference PolicyEngine's current YAML test shape, or
2. a small upstream docs/testing contribution if useful?

Given PolicyEngine's existing validation culture, the highest-value next step may be a narrow demonstration on a package slice rather than a broad format discussion.

Evidence packet (external): https://github.com/edithatogo/rulesandprocesses/tree/main/converters/fixtures

[edithatogo]

Tidied the issue body to remove internal draft framing and point evidence links at the public packet in edithatogo/rulesandprocesses. Substance of the proposal is unchanged.