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/discovery.test.ts

@@ -86,11 +86,16 @@ function makeFakeNg() {
   const sparql_query = mock(async (...a: unknown[]) => {
     const query = a[1] as string;
     const anchor = a[3] as string | undefined;
-    // Shim account SELECT.
+    // Shim account SELECT. Two shapes: the full scan (`?acc a <Account>`) and
+    // the TARGETED bounded resolve (`<subj> a <Account>`), which binds one
+    // subject — honour that subject filter so the bounded query is O(1)/exact.
     if (query.includes(`<${SHIM}:username>`)) {
+      const subjM = query.match(new RegExp(`GRAPH <[^>]+>\\s*\\{\\s*<([^>]+)>\\s+a\\s+<${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 === `${SHIM}:username`) rec.username = q.o;
         if (q.p === `${SHIM}:docPublic`) rec.docPublic = q.o;