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/ui-theme/tests/apply.spec.ts

@@ -10,6 +10,7 @@ import {
   apply, inject, SETTINGS_NS, THEME_SETTINGS_NAMESPACE,
 } from '@deepseek-ai/dsh-client-ui-theme/client'
 import type { AppearanceRowInjected, ThemeService } from '@deepseek-ai/dsh-client-ui-theme/client'
+import { ThemeSettingsSchema } from '@deepseek-ai/dsh-client-ui-theme'
 import { AppearanceRow } from '../src/client/AppearanceRow.tsx'
 import type { createAppearanceRowStore } from '../src/client/settings-store.ts'
 
@@ -33,7 +34,7 @@ async function bench(isLoopback = true) {
   let preference = 'system'
   const namespace = () => ({
     ns: THEME_SETTINGS_NAMESPACE,
-    schema: {},
+    schema: ThemeSettingsSchema.toJSON(),
     value: { preference },
     applies: 'live' as const,
     secrets: [],