Go SDK: the json index as a stash tag, with one generated query method per form #1061

Updated 2026-10-05 (PDT). The Go SDK design is PR cipherstash/stack#1070: docs/plans/2026-10-04-plan-builder.md, "The Go SDK", and the principles in docs/sdk-design-principles.md (ADR-0008). The declaration is a tag, stash:"attrs,index=json", with options in parentheses after the name. stashgen writes one query method on the field entry for each JSON query form. There is no Json() constructor and no plantest. The JSON index itself is a new engine operation, and the generator refuses index=json until the engine has it.

Background

The Go binding (languages/golang/stackencrypt/) runs stack-encrypt, our Rust field-level encryption library, inside a WebAssembly guest. #1046 reshapes it into one chained Encrypt that takes a plan: per field, a context and the search indexes to derive. #1060 adds a JSON index to stack-encrypt, so a JSON document field can be queried by containment, by path, and by value at a path.

Problem

Once the Rust JSON index exists, Go has no way to name it in a plan or to ask the three JSON query forms. Go cannot pick a query form from a static source type the way Rust does, so the forms need an explicit Go spelling.

Proposal

From docs/plans/2026-10-04-plan-builder.md (PR cipherstash/stack#1052), "The Go mirror". Exact signatures belong in the PR.

1. stackencrypt.Json() as an index, used as Index("attrs", stackencrypt.Json()) in a chain or saved plan, with its options (array index mode, Compat/Standard, case filters) as Go options on the constructor.
2. Query(..).Using(attrsPlan) with the form chosen by the source value's Go type: a document value for containment, a path value for a selector, a path-with-value for equality at a path.
3. Decrypting an extracted entry works through the same Decrypt(..).Using(..) call.
4. Live tests and plantest.Golden snapshots cover all three forms.

Relationship to other work

• Blocked by #1060 (the Rust JSON index) and #1046 (the Go chain).
• Design: docs/plans/2026-10-04-plan-builder.md on PR cipherstash/stack#1052.