Stack Encrypt refuses the JSON index in a data plan, so the Go SDK cannot seal a json_tree document #1349

Background

The Go SDK runs the Rust engine inside a WebAssembly guest. Go never encrypts anything itself. It describes each field in a data plan (the field's name, its type and its index), and the engine does the work. This is ADR-0007 and design principle G1, and it is why every language produces the same bytes.

Problem

The engine refuses the JSON index in a data plan. IndexSpec::Json parses, and it then fails with Reason::JsonIndexPending, "the JSON index has no data-plan form yet" (packages/stack-encrypt/src/dynamic/kind.rs, dynamic/mod.rs). Only typed Rust can seal a json_tree document today. Go, and any future language SDK, cannot.

Proposal

1. Admit IndexSpec::Json, with its options (sealed_root, ordered paths), in the data plan.
2. Add a json source kind whose value is JSON text.
   • The engine parses it with serde_json, so number handling (2 == 2.0) and key handling stay in Rust.
   • It then seals the value through the same layout the typed path uses.
   • Opening returns JSON text.
3. Teach the guest's EQL target resolver the family, so the json_tree types become producible.
4. Tests:
   • A parity test: the same document gives the same terms through the data plan as through typed Rust, and each path opens what the other sealed.
   • The mutants gate on the changed lines.

Depends on the catalog issue.

Plan: docs/plans/2026-10-24-go-json-tree.md, step 2.