feat(19.6.1-05): persist picked relation labels across FormView tab remount

- FORM_REMEMBER_LABEL inject so hosts keep picked option names
- RelationField choose() writes the label before emit
- Vitest backstops remount and tab-switch labeling

Co-authored-by: Cursor <cursoragent@cursor.com>

admin/src/components/form/formContext.ts

@@ -5,7 +5,7 @@
 // provider (settings pages) gets the defaults of the injecting control.
 import { ref, type InjectionKey, type Ref } from 'vue'
 import type { FileRoutes } from '../../api/files'
-import type { AdminDependentOptionsResult, AdminRecord, FormField, PermissionRecordLock } from '../../api/types'
+import type { AdminDependentOptionsResult, AdminRecord, FormField, PermissionRecordLock, RelationOption } from '../../api/types'
 
 /** Read-only view of the form's current values. */
 export const FORM_VALUES: InjectionKey<Readonly<Ref<AdminRecord>>> = Symbol('summer.form.values')
@@ -171,5 +171,13 @@ export interface FieldScope {
 
 export const FIELD_SCOPE: InjectionKey<FieldScope> = Symbol('summer.form.fieldScope')
 
+/**
+ * Form hosts persist a picked relation option so a remounted RelationField
+ * can seed its id→label cache from props.labels; a host without a provider
+ * is a no-op.
+ */
+export const FORM_REMEMBER_LABEL: InjectionKey<(field: string, option: RelationOption) => void> =
+  Symbol('summer.form.rememberLabel')
+
 /** Dependent options per field path and sibling value, for the life of a form view. */
 export const DEPENDENT_CACHE: InjectionKey<Map<string, AdminDependentOptionsResult>> = Symbol('summer.form.dependentCache')