T01.i: remove dead ensureReposOpen (batch open-repo helper)

Its only callers were its own definition and its dedicated test block;
no production code used it since the read-union removal (T01.f). The
one test that exercised readDocumentSubjects' real integration path
(not ensureReposOpen itself) is kept, moved under the ensureRepoOpen
describe block it actually belongs to.

packages/polyfill/src/emulated-verifier/open-repo.ts

@@ -285,14 +285,3 @@ export async function openRepoUnguarded(nuri: Nuri): Promise<void> {
   inFlight.set(nuri, p);
   return p;
 }
-
-/**
- * Open a SET of repos before an anchored batch read, in parallel, each tolerant
- * ({@link ensureRepoOpen} never throws). Empty / falsy entries are ignored.
- */
-// @provenance ensureReposOpen kind=aligned level=2 ref=sdk/js/lib-wasm/src/lib.rs:doc_subscribe — the batch form, with per-doc isolation
-export async function ensureReposOpen(nuris: Nuri[]): Promise<void> {
-  const unique = [...new Set(nuris.filter(Boolean))];
-  if (unique.length === 0) return;
-  await Promise.all(unique.map((n) => ensureRepoOpen(n)));
-}