refactor(client): replace the per-field settings preference controller with a namespace settings scope

bindSettingsScope mirrors the Host-side settings owner seam in the browser:
one scope per namespace publishes a snapshot store (status, section value,
revision, writability, host/memory mode), validates sections against the
namespace's serialized wire schema via dsh-client-schema-form, and keeps the
controller's listener-before-read, revisioned serialized writes, latest-wins
publication, conflict recovery, and disposal quiescence. Theme, locale, and
busy-Enter services now take the scope as a constructor collaborator, which
removes the bindPersistence/syncPreference two-phase callback pair and the
defaulted no-op persist writers; hand-written wire guards fall away in favor
of the registered schema. test-runtime gains a stubSettingsScope double.

packages/client/test-runtime/src/index.ts

@@ -34,6 +34,8 @@ import type { Stabilizer } from './fixtures.ts'
 
 export { domSnapshotSerializer, registerDomSnapshotSerializer } from './snapshot.ts'
 export { FixtureSession, TestSessions } from './sessions.ts'
+export { stubSettingsScope } from './settings-scope.ts'
+export type { StubSettingsScope } from './settings-scope.ts'
 export { TestWorkspaces } from './workspaces.ts'
 export { conversationSnapshot, workspaceListState } from './fixtures.ts'
 export type { SessionBehaviorOverrides, SessionFixture, Stabilizer } from './fixtures.ts'

packages/client/test-runtime/src/settings-scope.ts

@@ -0,0 +1,48 @@
+/** Test double for the client settings-scope seam. */
+import { vi } from 'vitest'
+import type { SettingsScope, SettingsScopeSnapshot } from '@deepseek-ai/dsh-client-runtime/client'
+
+/** Handle over one stubbed scope: the scope, its write spy, and publication controls. */
+export interface StubSettingsScope<T> {
+  /** The scope face handed to the service under test. */
+  scope: SettingsScope<T>
+  /** Spy behind `scope.set`; resolves immediately. */
+  set: ReturnType<typeof vi.fn>
+  /** @returns how many listeners are currently subscribed (disposal assertions). */
+  listenerCount(): number
+  /**
+   * Replace part of the snapshot and notify subscribers, as a Host
+   * acceptance would.
+   * @param next - snapshot fields to replace.
+   */
+  publish(next: Partial<SettingsScopeSnapshot<T>>): void
+}
+
+/**
+ * Build an in-memory settings scope for service specs: starts in the host
+ * loading state, records writes, and lets the test publish Host acceptances.
+ * @returns the stub handle.
+ */
+export function stubSettingsScope<T>(): StubSettingsScope<T> {
+  let snapshot: SettingsScopeSnapshot<T> = {
+    status: 'loading', value: undefined, revision: undefined, writable: false, mode: 'host',
+  }
+  const listeners = new Set<() => void>()
+  const set = vi.fn(() => Promise.resolve())
+  return {
+    scope: {
+      getSnapshot: () => snapshot,
+      subscribe: (listener) => {
+        listeners.add(listener)
+        return () => { listeners.delete(listener) }
+      },
+      set,
+    },
+    set,
+    listenerCount: () => listeners.size,
+    publish: (next) => {
+      snapshot = { ...snapshot, ...next }
+      for (const listener of [...listeners]) listener()
+    },
+  }
+}