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/src/index.ts

@@ -18,6 +18,8 @@ export * as inbox from "./inbox";
 export * as discovery from "./discovery";
 export type { IndexEntry, SubmitOptions } from "./discovery";
 export * as docs from "./docs";
+export { subscribeDoc, subscribeDocs } from "./subscribe";
+export type { DocChange, Unsubscribe } from "./subscribe";
 export * as readModel from "./read-model";
 export type { UnionSubject } from "./read-model";
 export * as storeRegistry from "./store-registry";