refactor(home): restructure extension architecture to match catalog pattern

Reorganize the home plugin's new frontend system extensions to follow
the same pattern as the catalog plugin's EntityContentLayoutBlueprint.
Key changes:
- Widgets now attach directly to page:home instead of through an
  intermediate HomepageBlueprint extension
- Add HomePageLayoutBlueprint in home-react for custom layout creation,
  with a built-in default layout as fallback
- Remove compatWrapper from HomepageWidgetBlueprint (consumer
  responsibility, reduced need with NFS support in old system)
- Remove HomepageCompositionRoot usage (old system artifact)
- Delete HomepageBlueprint (replaced by HomePageLayoutBlueprint)
- Update app-next example and dev harness to use new pattern
Signed-off-by: Adam Kunicki <kunickiaj@gmail.com>

plugins/home-react/src/alpha.ts

@@ -28,8 +28,14 @@ export {
   HomepageWidgetBlueprint,
   type HomepageWidgetBlueprintParams,
 } from './alpha/blueprints/HomepageWidgetBlueprint';
+export {
+  HomePageLayoutBlueprint,
+  type HomePageLayoutBlueprintParams,
+} from './alpha/blueprints/HomePageLayoutBlueprint';
 export {
   homePageWidgetDataRef,
   type HomePageWidgetData,
+  homePageLayoutComponentDataRef,
+  type HomePageLayoutProps,
 } from './alpha/dataRefs';
 export type { ComponentParts, CardLayout, CardSettings } from './extensions';

plugins/home-react/src/alpha/blueprints/HomePageLayoutBlueprint.tsx

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2025 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { JSX } from 'react';
+import {
+  createExtensionBlueprint,
+  ExtensionBoundary,
+} from '@backstage/frontend-plugin-api';
+import {
+  homePageLayoutComponentDataRef,
+  HomePageLayoutProps,
+} from '../dataRefs';
+
+/**
+ * Parameters for creating a home page layout extension.
+ *
+ * @alpha
+ */
+export interface HomePageLayoutBlueprintParams {
+  /**
+   * Async loader that returns the layout component.
+   * The component receives the collected widgets and renders them.
+   */
+  loader: () => Promise<(props: HomePageLayoutProps) => JSX.Element>;
+}
+
+/**
+ * Blueprint for creating custom home page layouts.
+ *
+ * A layout receives the list of installed widgets and is responsible for
+ * arranging them on the home page. This follows the same pattern as
+ * `EntityContentLayoutBlueprint` in the catalog plugin.
+ *
+ * If no layout extension is installed, the home page uses a built-in default.
+ * Users can install their own layout to customize how widgets are arranged.
+ *
+ * @alpha
+ */
+export const HomePageLayoutBlueprint = createExtensionBlueprint({
+  kind: 'home-page-layout',
+  attachTo: { id: 'page:home', input: 'layouts' },
+  output: [homePageLayoutComponentDataRef],
+  dataRefs: {
+    component: homePageLayoutComponentDataRef,
+  },
+  *factory({ loader }: HomePageLayoutBlueprintParams, { node }) {
+    yield homePageLayoutComponentDataRef(
+      ExtensionBoundary.lazyComponent(node, loader),
+    );
+  },
+});

plugins/home-react/src/alpha/blueprints/HomepageWidgetBlueprint.tsx

@@ -15,7 +15,6 @@
  */
 
 import { lazy, ReactElement } from 'react';
-import { compatWrapper } from '@backstage/core-compat-api';
 import {
   createExtensionBlueprint,
   ExtensionBoundary,
@@ -96,12 +95,11 @@ export const HomepageWidgetBlueprint = createExtensionBlueprint({
 
     const Widget = (
       props: CardExtensionProps<Record<string, unknown>>,
-    ): ReactElement =>
-      compatWrapper(
-        <ExtensionBoundary node={node}>
-          <LazyCard {...props} />
-        </ExtensionBoundary>,
-      );
+    ): ReactElement => (
+      <ExtensionBoundary node={node}>
+        <LazyCard {...props} />
+      </ExtensionBoundary>
+    );
 
     yield homePageWidgetDataRef({
       component: <Widget {...(params.componentProps ?? {})} />,

plugins/home-react/src/alpha/dataRefs.ts

@@ -15,7 +15,7 @@
  */
 
 import { createExtensionDataRef } from '@backstage/frontend-plugin-api';
-import { ReactElement } from 'react';
+import { JSX, ReactElement } from 'react';
 import type { CardLayout, CardSettings } from '../extensions';
 
 /**
@@ -64,3 +64,30 @@ export const homePageWidgetDataRef =
   createExtensionDataRef<HomePageWidgetData>().with({
     id: 'home.widget.data',
   });
+
+/**
+ * Props provided to a home page layout component.
+ *
+ * @alpha
+ */
+export interface HomePageLayoutProps {
+  /**
+   * The list of widget elements and metadata to render on the home page.
+   */
+  widgets: Array<HomePageWidgetData>;
+}
+
+/**
+ * Extension data ref for home page layout components.
+ *
+ * A layout receives the collected widgets and is responsible for arranging
+ * them on the home page. This follows the same pattern as
+ * EntityContentLayoutBlueprint in the catalog plugin.
+ *
+ * @alpha
+ */
+export const homePageLayoutComponentDataRef = createExtensionDataRef<
+  (props: HomePageLayoutProps) => JSX.Element
+>().with({
+  id: 'home.layout.component',
+});