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/src/app/winterUrl.ts

@@ -1,11 +1,12 @@
 // Winter-shaped URLs (config_list recordUrl, config_form redirects) mapped
 // onto the SPA's D-10 routes. The strings come from plugin YAML, so they are
-// never used verbatim: only the current controller's list, create and record
-// routes can come out (research Gap 8, T-10-20).
+// never used verbatim: only the current controller's list, create, record
+// and preview routes can come out (research Gap 8, T-10-20, T-12.1-16).
 //
 //   <vendor>/<plugin>/<controller>              -> list
 //   <vendor>/<plugin>/<controller>/create       -> create
 //   <vendor>/<plugin>/<controller>/update/:id   -> record (:id substituted)
+//   <vendor>/<plugin>/<controller>/preview/:id  -> preview (:id substituted)
 //   anything else, or another controller        -> list
 import { controllerPath, parseControllerId } from './controllerRoutes'
 
@@ -35,5 +36,9 @@ export function mapWinterUrl(url: string | null | undefined, controllerId: strin
     const target = rest[1] === ':id' ? String(id ?? '') : (rest[1] ?? '')
     return DIGITS.test(target) ? `${base}/${target}` : base
   }
+  if (rest.length === 2 && rest[0] === 'preview') {
+    const target = rest[1] === ':id' ? String(id ?? '') : (rest[1] ?? '')
+    return DIGITS.test(target) ? `${base}/${target}/preview` : base
+  }
   return base
 }