📝 Document the protocol-backed LSP helper

Note that the autocomplete helper drives the server through
vscode-languageserver-protocol, cross-link the tooling decision, and replace
the obsolete string-id known issue with the benign context-canceled stderr
the Go server emits on shutdown.

development/testing.md

@@ -134,7 +134,10 @@ The script prints the labels the server offers at a `/*COMPLETE*/` marker inside
 is imported by `src/util/__tests__/lsp-completion.test.ts` — which tests the
 helper itself against inline documents, never the library's code — and by
 `src/primitive.test.ts`, where the same probe asserts the matcher's popup;
-the CLI is for manual inspection.
+the CLI is for manual inspection. It speaks to the server through
+`vscode-languageserver-protocol`'s message connection and typed requests, so the
+transport and protocol plumbing are not hand-rolled (see
+[tooling.md § `vscode-languageserver-protocol` backs the autocomplete helper](./tooling.md#vscode-languageserver-protocol-backs-the-autocomplete-helper)).
 
 #### Why
 
@@ -162,8 +165,10 @@ the CLI is for manual inspection.
   any order; the file is an integration test (~1.6 s) that needs `node_modules`.
   `didOpen` is handled in order before the completion request, so no settle
   delay is needed.
-- The server answers some requests with a string id (`client/registerCapability`);
-  the client must tolerate `string | number` ids or the server stalls.
+- The Go server logs `context canceled` on stderr once per server as it shuts
+  down (nine lines per `test:unit` run). It is benign and predates the protocol
+  client — the hand-rolled client produced the same line — so it is left as is
+  rather than filtered out of the server's stderr.
 
 ## Known issues