feat(12.1-02): read-only preview screen with a status hint and record actions

- config_form.yaml preview block (optional headerPartial), reported in the form schema as preview
- fields with context: preview show only on the preview screen and are never written
- form messages preview and edit; recordActions without a preview block stops boot
- SPA route {id}/preview, PreviewView and PreviewField, record actions in the footer
- mapWinterUrl maps preview/:id; the update form returns to the preview
- summer-callout partial style classes for status hints
- README, docs, OpenAPI document, TS types and the embedded build updated

admin/tests/fixtures/roster.list-schema.json

@@ -8,7 +8,7 @@
     "showCheckboxes": true,
     "showSorting": true,
     "searchTerm": "search",
-    "recordUrl": "acme/roster/people/update/:id",
+    "recordUrl": "acme/roster/people/preview/:id",
     "toolbarButtons": ["create", "delete"],
     "toolbarActions": [],
     "assets": { "scripts": [], "styles": [] },