feat(client): per-document reactive subscription (doc_subscribe), drop polling

Expose subscribeDoc(nuri, onChange) / subscribeDocs(nuris, onChange) wrapping the
real ng.doc_subscribe — per-document, event-driven (initial State + a Patch per
commit, local or broker-synced from a remote peer), returning a sync unsubscribe.
subscribeDocs isolates per doc (a failing/unsynced doc never aborts the others),
so it sidesteps the ORM fan-out hang (never orm_start_graph(graphs:[…])).

Replace the setInterval polling in inbox.watch() and discovery.watchIndex() with
doc_subscribe — same public contract, now push not poll. NextGraph is subscription-
first; no polling remains.

Verified against the real broker (@data harness spike): the doc_subscribe callback
marshals across the iframe RPC (@ng-org/web strips the callback and drives it via a
MessagePort — no DataCloneError) and fires on the initial push and on a real write.
Lib bun test 91 pass, tsc clean.

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

packages/client/test/discovery.test.ts

@@ -57,6 +57,25 @@ function makeFakeNg() {
   const quads: Quad[] = [];
   let docCounter = 0;
 
+  // Reactive subscriptions (see inbox.test.ts): doc_subscribe registers a
+  // callback per anchor + fires an initial push; sparql_update pushes a Patch to
+  // that anchor's subscribers, so discovery.watchIndex (now event-driven) works
+  // without a timer.
+  const subs = new Map<string, Set<(r: unknown) => void>>();
+  const doc_subscribe = mock(async (nuri: string, _sid: unknown, cb: (r: unknown) => void) => {
+    let set = subs.get(nuri);
+    if (!set) {
+      set = new Set();
+      subs.set(nuri, set);
+    }
+    set.add(cb);
+    queueMicrotask(() => cb({ V0: { State: { doc: nuri } } }));
+    return () => set!.delete(cb);
+  });
+  const pushTo = (anchor: string): void => {
+    for (const cb of subs.get(anchor) ?? []) cb({ V0: { Patch: { doc: anchor } } });
+  };
+
   const doc_create = mock(async (..._a: unknown[]) => `did:ng:o:doc${++docCounter}`);
 
   const sparql_update = mock(async (...a: unknown[]) => {
@@ -93,6 +112,7 @@ function makeFakeNg() {
       const o = m[2] !== undefined ? unescapeLiteral(m[2]) : (m[3] ?? "");
       quads.push({ g, s, p, o });
     }
+    pushTo(g); // local-push to the written graph's subscribers
     return undefined;
   });
 
@@ -157,7 +177,7 @@ function makeFakeNg() {
     return { results: { bindings: [] } };
   });
 
-  return { doc_create, sparql_update, sparql_query, _quads: quads };
+  return { doc_create, doc_subscribe, sparql_update, sparql_query, _quads: quads };
 }
 
 const SESSION: RegistrySession = { sessionId: "sid-1", privateStoreId: "PRIV" };