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/src/theme-settings.ts

@@ -15,6 +15,12 @@ export type ThemePreference = typeof THEME_PREFERENCES[number]
 /** Default preference when the user-settings document has no override. */
 export const DEFAULT_PREFERENCE: ThemePreference = 'system'
 
+/** Durable theme section shared by the Host schema and the browser scope. */
+export interface ThemeSettings {
+  /** Selected built-in preference. */
+  preference: ThemePreference
+}
+
 /**
  * Narrow one wire or registry value to a persistable preference.
  * @param value - value crossing the settings or registry boundary.