docs(stack-encrypt): a guide to building a language integration

Nothing explained how to build a binding on Stack Encrypt. The knowledge
was spread over the dynamic module's docs, the Go guest's module docs,
stack-guest-abi, ADR-0007 and ADR-0008, and an author had to reconstruct
it from the Go SDK. A Python integration has started without it.

The guide ships in the crate docs as `dynamic::guide`, included from
`docs/language-integration.md`, so it renders on docs.rs beside the API
it describes. It is ordered contract before API: the binding versus the
language SDK; native versus WebAssembly; the three operations and the
Go guest's exports; the plan grammar; control messages versus plaintext,
both frames, neither a tree; the plaintext hygiene rules and the heap
tests that prove them; the step loop, the token import and the
cancellation contract; the status table and se_last_error; versioning
and the frozen stored-rows fixtures; one int32 stored as EQL v4
IntegerEq, traced through encrypt, query and decrypt with the bytes from
the committed fixtures; and a checklist for a new binding. The frame
format stays in vitaminc-aead-frame's documentation and is linked, not
restated.

The frame bytes in the worked example are asserted by doctests, so the
guide fails the build if the engine's bytes move. The stored row and the
query term are quoted from fixtures the Go SDK and Rust both read, named
by file and key.

Linked from the crate docs, the SDK design principles and the Go SDK's
README.

Closes #1254.

Author: coderdan

docs/sdk-design-principles.md

@@ -13,6 +13,8 @@ The second part applies to the Go SDK.
 - **Declaration:** the statement of how each field is encrypted: its context, and its indexes.
   The engine calls a saved declaration a plan.
 
+How to build the binding itself, what crosses it and how to prove it, is the guide in `packages/stack-encrypt/docs/language-integration.md` (`stack_encrypt::dynamic::guide` in the crate documentation).
+
 ## When two principles disagree
 
 Apply them in this order:

languages/golang/encrypt/README.md

@@ -588,6 +588,10 @@ credential guest) before `go test`, and
 round-trip and fixture tests use. Without the builds `NewClient` returns
 `ErrGuestNotBuilt` and the tests skip.
 
+This package is the reference integration for the engine. How the binding
+works, what crosses it and how to build one for another language is the
+guide in `packages/stack-encrypt/docs/language-integration.md`.
+
 ## Licence
 
 The Go code in this module is MIT-licensed ([`LICENSE`](../LICENSE)). The

packages/stack-encrypt/CHANGELOG.md

@@ -9,6 +9,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- **A guide to building a language integration**, `dynamic::guide`
+  (`docs/language-integration.md`, shipped in the crate docs under the
+  `dynamic` feature): the binding versus the language SDK, native versus
+  WebAssembly, the three operations and the Go guest's exports, the plan
+  grammar, control messages versus plaintext, the plaintext hygiene rules
+  and the tests that prove them, the step loop and the token import,
+  the status table and `se_last_error`, versioning, one `int32` stored as
+  EQL v4 `IntegerEq` traced through encrypt, query and decrypt with the
+  bytes from the committed fixtures, and a checklist for a new binding.
+  The frame format stays in `vitaminc-aead-frame`'s documentation and is
+  linked, not restated.
 - **Every integer width, 128-bit integers, dates, timestamps and decimals
   as dynamic values** (`dynamic` feature; #1144, ADR-0002). They are
   vitaminc leaves in a `ValueFrame`, with no new `FfiValue` variant: