feat(client-runtime): carry the layered view and a field reset through the settings scope

A form needs two things the snapshot did not carry. The `user` layer tells it
which fields the user overrode — presence, not value equality, because an
override equal to the composition default is still an override — and `base`
is what a cleared field reverts to. `unset` is that clear, sharing `set`'s
queue, revision fence, and rejected-write recovery through one write path.

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

@@ -8,6 +8,8 @@ export interface StubSettingsScope<T> {
   scope: SettingsScope<T>
   /** Spy behind `scope.set`; resolves immediately. */
   set: ReturnType<typeof vi.fn>
+  /** Spy behind `scope.unset`; resolves immediately. */
+  unset: ReturnType<typeof vi.fn>
   /** @returns how many listeners are currently subscribed (disposal assertions). */
   listenerCount(): number
   /**
@@ -25,10 +27,12 @@ export interface StubSettingsScope<T> {
  */
 export function stubSettingsScope<T>(): StubSettingsScope<T> {
   let snapshot: SettingsScopeSnapshot<T> = {
-    status: 'loading', value: undefined, revision: undefined, writable: false, mode: 'host',
+    status: 'loading', value: undefined, base: undefined, user: undefined,
+    revision: undefined, writable: false, mode: 'host',
   }
   const listeners = new Set<() => void>()
   const set = vi.fn(() => Promise.resolve())
+  const unset = vi.fn(() => Promise.resolve())
   return {
     scope: {
       getSnapshot: () => snapshot,
@@ -37,8 +41,10 @@ export function stubSettingsScope<T>(): StubSettingsScope<T> {
         return () => { listeners.delete(listener) }
       },
       set,
+      unset,
     },
     set,
+    unset,
     listenerCount: () => listeners.size,
     publish: (next) => {
       snapshot = { ...snapshot, ...next }