📝 Check off the LSP helper task

npm run verify is green: 23 tests pass, check:tsc, check:oxlint, check:oxfmt
and check:cspell clean with the protocol-backed helper.

backlog.tasks

@@ -32,11 +32,11 @@ Testing:
     → `src/util/__tests__/lsp-completion.ts` (`#test-utils/…`) is the helper; the suite asserts its labels (see development/testing.md § Autocomplete)
     → wire it into `node --test` so a test asserts the offered labels
     → note: `Parameters<typeof factory>[0]` resolves only the *last* overload; use `@ts-expect-error` call sites for factory negatives, not `not.toExtend<Parameters<…>>`
-☐ Drive the LSP completion helper through the LSP protocol library instead of a hand-rolled JSON-RPC client @medium
-    ☐ Add `vscode-languageserver-protocol` and record the tooling decision
-    ☐ Rewrite `src/util/__tests__/lsp-completion.ts` onto `createMessageConnection` and typed requests
-    ☐ Update `development/testing.md § Autocomplete` for the new client
-    ☐ Run `npm run verify` and check the task off
+✔ Drive the LSP completion helper through the LSP protocol library instead of a hand-rolled JSON-RPC client @medium @done
+    ✔ Add `vscode-languageserver-protocol` and record the tooling decision @done
+    ✔ Rewrite `src/util/__tests__/lsp-completion.ts` onto `createMessageConnection` and typed requests @done
+    ✔ Update `development/testing.md § Autocomplete` for the new client @done
+    ✔ Run `npm run verify` and check the task off @done
 
 Matcher:
 ✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done