feat(client): read-model surface — open/sync + anchorless union sparql_query

Proven against the real broker (probe): opening docs then a single anchorless
`sparql_query` with a `GRAPH ?g { ... }` body reads the LOCAL UNION of all synced
named graphs — the fast, hang-free replacement for the reactive-ORM per-document
fan-out (which aborted on any unsynced/fresh repo → 75s never-fires). New
`read-model.ts` (readUnion) exposes this; there is no reactive union query, so
listing is one-shot and consumers re-query on change. docs/read-model.md refined
with the probe finding (an explicit `GRAPH ?g` body iterates all named graphs
regardless of the anchor; the anchor only bounds the default graph). 93 tests
pass; tsc rc=0.

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 * as readModel from "./read-model";
+export type { UnionSubject } from "./read-model";
 export * as storeRegistry from "./store-registry";
 export type { AccountRecord, RegistrySession } from "./store-registry";
 export * as isolation from "./isolation";