perf(client): targeted shim account resolution (no full-scan on hot paths)

loadShim() read EVERY account record in the shim (SELECT over the whole anchor
graph). On a shim that accumulates accounts (the shared wallet grows), that is
O(accounts) and hangs the hot path (~90s at ensureAccount → loadShim). Same
principle as per-doc reads: never scan a shared structure.

Add resolveAccount(username): a BOUNDED SELECT anchored on the single subject
accountSubject(username) → O(1), independent of account count. Cache in a
per-account Map (cleared by resetRegistryCache). Hot paths now use it:
ensureAccount (existence check), indexInboxNuri/@index (discovery), resolveInbox
Anchor, resolveWriteGraph, createEntityDoc, listMyEntityDocs. loadShim kept only
for genuine all-accounts needs (allAccounts, the listEntityDocs fan-out fallback).

The 90s ensureAccount/loadShim hang is gone. 93 tests pass; tsc rc=0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/client/test/store-registry.test.ts

@@ -97,10 +97,16 @@ function makeFakeNg() {
     const query = a[1] as string;
     const anchor = a[3] as string | undefined;
     if (query.includes("<urn:ng-eventually:shim:username>")) {
-      // Account SELECT, scoped to the anchor graph.
+      // Account SELECT, scoped to the anchor graph. Two shapes: the full scan
+      // (`?acc a <Account>`) and the TARGETED bounded resolve (`<subj> a
+      // <Account>`) which binds one subject — honour that subject so the bounded
+      // query returns exactly that account (or nothing).
+      const subjM = query.match(/GRAPH <[^>]+>\s*\{\s*<([^>]+)>\s+a\s+<urn:ng-eventually:shim:Account>/);
+      const onlySubject = subjM ? subjM[1]! : null;
       const bySubject = new Map<string, Record<string, string>>();
       for (const q of quads) {
         if (q.g !== anchor) continue;
+        if (onlySubject !== null && q.s !== onlySubject) continue;
         const rec = bySubject.get(q.s) ?? {};
         if (q.p === "urn:ng-eventually:shim:username") rec.username = q.o;
         if (q.p === "urn:ng-eventually:shim:docPublic") rec.docPublic = q.o;