feat: add support for params

[stefan-gorules]

No description provided.

[phenixrizen]

Is there anything blocking this one? Happy to help if so.

We run a large production deployment of JDM graphs for healthcare claims adjudication, and a substantial share of them carry a per-engine sidecar file of policy constants — code sets, effective dates, allowed modifier lists. Today our host merges that into the evaluation input, which means a policy and the constants it depends on are two separate artifacts that can version-drift independently. That has been a recurring source of authoring problems for us.

$params fixes exactly that: the constants live in the decision document, so the policy travels with its own data. It would also let us replace a number of hand-rolled custom nodes with plain expressions like contains($params.codes, clue.procedureCode).

Two things we can offer, both inside what the contribution policy allows:

• We can run this branch against our full corpus and report back on real graphs — useful signal on the $params reserved-key handling and the trace/transform stripping in particular.
• We're glad to write additional test coverage or documentation for it if that's what's outstanding.

Not asking to jump the queue — just wanted to flag that there's a real user waiting on it, and to offer hands if the remaining work is the kind we can pick up.