refactor(client-runtime): home the snapshot-store engine in contract/

The store migration (#538) parked the engine in a store/ sibling domain;
loader and sessions imported it directly, tripping verify-client-domain-graph
(cross-domain shared surface must route through contract/). The engine is
layer-0 shared infrastructure by definition, so it moves to contract/store.ts
verbatim; the four importers and the spec follow mechanically.

packages/client/runtime/tests/store.spec.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest'
-import { createSnapshotStore, defineStore, shallowEqual } from '../src/client/store/index.ts'
+import { createSnapshotStore, defineStore, shallowEqual } from '../src/client/contract/store.ts'
 
 interface State {
   a: { n: number }