docs: add preamble documentation for plugin-react-swc

- Add preamble section to plugin-react-swc README
- Update CHANGELOG entries for both plugin-react and plugin-react-swc
- Document the new virtual module feature for SSR HMR setup

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: hi-ogawa

packages/plugin-react-swc/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Add `@vitejs/plugin-react-swc/preamble` virtual module for SSR HMR ([#890](https://github.com/vitejs/vite-plugin-react/pull/890))
+
+SSR applications can now initialize HMR runtime by importing `@vitejs/plugin-react-swc/preamble` at the top of their client entry instead of manually calling `transformIndexHtml`. This simplifies SSR setup for applications that don't use the `transformIndexHtml` API.
+
 ## 4.1.0 (2025-09-17)
 
 ### Set SWC cacheRoot options

packages/plugin-react-swc/README.md

@@ -125,6 +125,38 @@ If set, disables the recommendation to use `@vitejs/plugin-react-oxc` (which is
 react({ disableOxcRecommendation: true })
 ```
 
+## `@vitejs/plugin-react-swc/preamble`
+
+The package provides `@vitejs/plugin-react-swc/preamble` to initialize HMR runtime from client entrypoint for SSR applications which don't use [`transformIndexHtml` API](https://vite.dev/guide/api-javascript.html#vitedevserver). For example:
+
+```js
+// [entry.client.js]
+import '@vitejs/plugin-react-swc/preamble'
+```
+
+Alternatively, you can manually call `transformIndexHtml` during SSR, which sets up equivalent initialization code. Here's an example for an Express server:
+
+```js
+app.get('/', async (req, res, next) => {
+  try {
+    let html = fs.readFileSync(path.resolve(root, 'index.html'), 'utf-8')
+
+    // Transform HTML using Vite plugins.
+    html = await viteServer.transformIndexHtml(req.url, html)
+
+    res.send(html)
+  } catch (e) {
+    return next(e)
+  }
+})
+```
+
+Otherwise, you'll get the following error:
+
+```
+Uncaught Error: @vitejs/plugin-react-swc can't detect preamble. Something is wrong.
+```
+
 ## Consistent components exports
 
 For React refresh to work correctly, your file should only export React components. The best explanation I've read is the one from the [Gatsby docs](https://www.gatsbyjs.com/docs/reference/local-development/fast-refresh/#how-it-works).

packages/plugin-react/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Unreleased
 
+### Add `@vitejs/plugin-react/preamble` virtual module for SSR HMR ([#890](https://github.com/vitejs/vite-plugin-react/pull/890))
+
+SSR applications can now initialize HMR runtime by importing `@vitejs/plugin-react/preamble` at the top of their client entry instead of manually calling `transformIndexHtml`. This simplifies SSR setup for applications that don't use the `transformIndexHtml` API.
+
 ## 5.0.4 (2025-09-27)
 
 ### Perf: use native refresh wrapper plugin in rolldown-vite ([#881](https://github.com/vitejs/vite-plugin-react/pull/881))