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/tests/form/FormView.test.ts

@@ -13,6 +13,7 @@ import {
   langFixture,
   listFixture,
   listSchemaFixture,
+  optionsFixture,
   recordFixture,
   rosterFormSchemaFixture,
   rosterRecordFixture,
@@ -931,3 +932,31 @@ describe('field triggers on the form (D-21, UI-SPEC C6 backstops)', () => {
     expect(banner.text()).toContain('Overrides are refused.')
   })
 })
+
+describe('picked relation labels across tab remount', () => {
+  type Mounted = Awaited<ReturnType<typeof mountApp>>['wrapper']
+
+  async function tab(wrapper: Mounted, key: string): Promise<void> {
+    await wrapper.find(`[role="tab"][data-tab="${key}"]`).trigger('click')
+    await flushPromises()
+  }
+
+  it('keeps a picked relation label after switching tabs', async () => {
+    const { wrapper } = await mountApp(
+      '/acme/demo/widgets/1',
+      routes({ [`GET ${BASE}/fields/maker/options`]: { body: optionsFixture.makerField } }),
+    )
+    const maker = () => wrapper.find('[data-field="maker"] [role="combobox"]')
+    expect((maker().element as HTMLInputElement).value).toBe('Bolt & Co')
+
+    await maker().trigger('click')
+    await flushPromises()
+    await wrapper.find('[data-field="maker"] [role="option"][data-value="1"]').trigger('mousedown')
+    await flushPromises()
+
+    await tab(wrapper, 'tab:Details')
+    await tab(wrapper, 'default')
+    expect((maker().element as HTMLInputElement).value).toBe('Acme Works')
+    expect((maker().element as HTMLInputElement).value).not.toBe('1')
+  })
+})