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/views/FormView.vue

@@ -4,7 +4,7 @@ import { onBeforeRouteLeave, onBeforeRouteUpdate, useRoute, useRouter } from 'vu
 import { ArrowLeft, Trash2 } from '@lucide/vue'
 import { api } from '../api/client'
 import { parentFileRoutes } from '../api/files'
-import type { AdminRecord, ErrorEnvelope, FormView, RecordEnvelope, RecordMeta } from '../api/types'
+import type { AdminRecord, ErrorEnvelope, FormView, RecordEnvelope, RecordMeta, RelationOption } from '../api/types'
 import { controllerIdFromParams, controllerPath } from '../app/controllerRoutes'
 import { currentLocale, message, t } from '../app/i18n'
 import { loadControllerAssets } from '../app/pluginAssets'
@@ -21,6 +21,7 @@ import {
   FORM_LOCALE,
   FORM_PATCH,
   FORM_PERMISSION_LOCKS,
+  FORM_REMEMBER_LABEL,
   FORM_SESSION,
   FORM_TRIGGER_NOTES,
   FORM_VALUES,
@@ -247,6 +248,18 @@ function adopt(record: RecordEnvelope | undefined): void {
   saved.value = snapshot(editablePayload(fields.value, values.value, mode, permissionLocks.value))
 }
 
+function rememberRelationLabel(field: string, option: RelationOption): void {
+  const current = labels.value[field] ?? []
+  const index = current.findIndex((item) => item.value === option.value)
+  const next = [...current]
+  if (index >= 0) {
+    next[index] = option
+  } else {
+    next.push(option)
+  }
+  labels.value = { ...labels.value, [field]: next }
+}
+
 async function load(): Promise<void> {
   loading.value = true
   failed.value = false
@@ -336,6 +349,7 @@ function update(name: string, value: unknown): void {
 // that field's errors like an edit (D-07, D-08).
 provide(FORM_VALUES, computed(() => values.value))
 provide(FORM_PATCH, update)
+provide(FORM_REMEMBER_LABEL, rememberRelationLabel)
 provide(FORM_LOCALE, computed(() => schema.value?.meta.locale || currentLocale.value))
 provide(FORM_ENABLED_LOCALES, computed(() => schemaEnabledLocales(schema.value?.meta)))
 provide(FORM_ASSETS, () => assetsReady)