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-conversation/src/submission-settings.ts

@@ -15,11 +15,8 @@ export type BusyEnterBehavior = typeof BUSY_ENTER_BEHAVIORS[number]
 /** Default preserves Enter-as-Queue for running conversations. */
 export const DEFAULT_BUSY_ENTER_BEHAVIOR: BusyEnterBehavior = 'queue'
 
-/**
- * Narrow one settings-wire value to a busy-Enter behavior.
- * @param value - value crossing the settings boundary.
- * @returns whether the value names a supported behavior.
- */
-export function isBusyEnterBehavior(value: unknown): value is BusyEnterBehavior {
-  return BUSY_ENTER_BEHAVIORS.some(behavior => behavior === value)
+/** Durable conversation section shared by the Host schema and the browser scope. */
+export interface ConversationSettings {
+  /** Delivery mode for plain Enter while the addressed agent is busy. */
+  busyEnter: BusyEnterBehavior
 }