feat(16.2-01): host plugin field type elements in the admin SPA

- loadFieldTypes fetches GET /field-types after sign-in, loads each script
  once and adds the styles as global links activateStyles never disables
- PluginField creates the type's element, sets the value property and the
  contract attributes, takes summer-change as the form value and never
  writes back a value the element just emitted
- rendererForField and isRegisteredField route any field with an element
  to PluginField in forms, previews and the save payload
- rebuilt modules/boardwalk/dist

admin/src/app/pluginAssets.ts

@@ -7,7 +7,10 @@
 // every navigation to another controller or to a screen that is not a
 // controller view, so plugin CSS never styles a view it was not written for.
 // Only the router picks the enabled controller; a schema response that
-// arrives after its view was left adds its links disabled.
+// arrives after its view was left adds its links disabled. Stylesheets of
+// plugin field types are global instead (GLOBAL_STYLE_OWNER): a field type's
+// element can appear in any controller's form, so its links are never
+// disabled on navigation.
 // Files load only through script and link elements; nothing here fetches.
 import type { ControllerAssets } from '../api/types'
 import { runtime } from './runtime'
@@ -15,6 +18,9 @@ import { runtime } from './runtime'
 /** Attribute naming the controller that owns a plugin stylesheet link. */
 export const OWNER_ATTRIBUTE = 'data-summer-controller'
 
+/** Owner of stylesheet links that stay enabled on every screen (field types). */
+export const GLOBAL_STYLE_OWNER = '*'
+
 const scripts = new Map<string, Promise<void>>()
 // Links by controller and URL: two controllers of one plugin may share a
 // file, and each keeps its own link so disabling one never hides the other.
@@ -90,7 +96,7 @@ export function loadStyles(controllerId: string, urls: readonly string[]): void
     const link = document.createElement('link')
     link.rel = 'stylesheet'
     link.setAttribute(OWNER_ATTRIBUTE, controllerId)
-    link.disabled = controllerId !== active
+    link.disabled = !styleEnabled(controllerId)
     link.href = url
     styles.set(key, { owner: controllerId, link })
     document.head.appendChild(link)
@@ -105,10 +111,23 @@ export function loadStyles(controllerId: string, urls: readonly string[]): void
 export function activateStyles(controllerId: string): void {
   active = controllerId
   for (const { owner, link } of styles.values()) {
-    link.disabled = owner !== active
+    link.disabled = !styleEnabled(owner)
   }
 }
 
+/** Whether links of this owner are enabled now: the active controller's and the global ones. */
+function styleEnabled(owner: string): boolean {
+  return owner === GLOBAL_STYLE_OWNER || owner === active
+}
+
+/**
+ * Adds stylesheet links that stay enabled on every screen: the styles of
+ * plugin field types, whose elements may render in any controller's form.
+ */
+export function loadGlobalStyles(urls: readonly string[]): void {
+  loadStyles(GLOBAL_STYLE_OWNER, urls)
+}
+
 /**
  * Adds the controller's stylesheet links and loads its scripts. The links
  * are enabled only while the router shows that controller, so a response