feat(client): discovery via a global index (special @index account)

Add a generic discovery-index surface: submitToIndex(ref) deposits a reference
into the index document's inbox; readIndex() returns the materialized entries. A
reserved special account (@index) owns the index document; deposits flow through
the emulated inbox and are materialized by the emulated curator (the dedup/
moderation point). This replaces cross-account fan-out as the discovery path and
is more faithful to the target (a single owned index fed via its inbox). Generic
(the consumer supplies the reference to index). 79 tests pass; tsc rc=0.

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

packages/client/src/index.ts

@@ -15,6 +15,8 @@ export * from "./types";
 export { useShape } from "./use-shape";
 export { init, initNg } from "./lifecycle";
 export * as inbox from "./inbox";
+export * as discovery from "./discovery";
+export type { IndexEntry, SubmitOptions } from "./discovery";
 export * as docs from "./docs";
 export * as storeRegistry from "./store-registry";
 export type { AccountRecord, RegistrySession } from "./store-registry";