feat: keep dev server alive during HMR updates

packages/dev-server/src/watch.ts

@@ -3,10 +3,9 @@
  * cheapest update that still shows the latest page:
  *
  *   *.css / styles/   -> invalidate CSS cache, push { type: "css" }  (instant swap)
- *   anything else      -> a server module changed (pages, components, api, …):
- *                         it can't be re-imported in process, so request a
- *                         restart (the supervisor respawns us; the browser then
- *                         morphs in the new HTML).
+ *   anything else      -> invalidate changed modules, rescan routes, and ask
+ *                         the connected browser to sync fresh HTML. The server
+ *                         process and HMR socket stay alive.
  */
 
 import { watch, type FSWatcher } from "node:fs";
@@ -17,14 +16,15 @@ export interface WatchOptions {
   appDir: string;
   hub: HmrHub;
   assets: DevAssetServer;
-  /** Called when a change requires a fresh process. */
-  onServerChange: () => void;
+  /** Called with changed app-relative files that need an in-process hot update. */
+  onHotChange: (files: string[]) => void | Promise<void>;
 }
 
 function isIgnored(rel: string): boolean {
   return (
     rel.includes("node_modules/") ||
     rel.includes(".wrnexus/") ||
+    rel.includes(".wrnexus-hmr-") ||
     rel.startsWith("dist/") ||
     rel.includes("/dist/")
   );
@@ -37,19 +37,22 @@ function classify(rel: string): Kind {
   return "server";
 }
 
-/** Returns the watcher so the caller can close it before a restart (important on
- * Windows, where a live recursive fs.watch handle can block `process.exit`). */
+/** Returns the watcher so the running server can close it during shutdown. */
 export function startWatcher(opts: WatchOptions): FSWatcher | undefined {
-  const { appDir, hub, assets, onServerChange } = opts;
+  const { appDir, hub, assets } = opts;
   const pending = new Set<Kind>();
+  const pendingFiles = new Set<string>();
   let timer: ReturnType<typeof setTimeout> | null = null;
 
   const flush = (): void => {
     timer = null;
-    // A server change always wins (needs a restart).
     if (pending.has("server")) {
       pending.clear();
-      onServerChange();
+      const files = [...pendingFiles];
+      pendingFiles.clear();
+      void Promise.resolve(opts.onHotChange(files)).catch((error) => {
+        console.error("[wrnexus] hot update failed", error);
+      });
       return;
     }
     if (pending.has("css")) {
@@ -57,6 +60,7 @@ export function startWatcher(opts: WatchOptions): FSWatcher | undefined {
       hub.css();
     }
     pending.clear();
+    pendingFiles.clear();
   };
 
   try {
@@ -64,6 +68,7 @@ export function startWatcher(opts: WatchOptions): FSWatcher | undefined {
       if (!filename) return;
       const rel = filename.toString().replace(/\\/g, "/");
       if (isIgnored(rel)) return;
+      pendingFiles.add(rel);
       pending.add(classify(rel));
       if (timer) clearTimeout(timer);
       timer = setTimeout(flush, 60); // debounce editor write bursts