fix: suppress route reload events during the initial route scan

The first dev request triggers buildRoutes(), which emitted a "reload"
event per discovered file. The fs-watcher's debounced handlers then
invalidated the routes manifest ~200ms after it had already been served,
pushing an HMR update that re-imported solid-start:routes, router.jsx,
and the user's app.tsx as fresh module instances while hydration was
still in flight. Depending on where the update landed, dev sessions saw
duplicated pages, dead client-side navigation, or actions whose handlers
lived in the stale module instance. Cold-start reproduction fired on
roughly half of runs; with the scan quiet it no longer fires at all,
while edits and added/removed route files still reload as before.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: brenelz

.changeset/quiet-initial-route-scan.md

@@ -0,0 +1,5 @@
+---
+"@solidjs/start": patch
+---
+
+Don't emit route "reload" events during the initial file-system route scan. The scan runs on the first dev request, and the events invalidated the just-served routes manifest ~200ms later, pushing a spurious HMR update of the app/router module chain that raced hydration — intermittently duplicating pages, breaking client-side navigation, and detaching actions in dev.

packages/start/src/config/fs-routes/router.ts

@@ -55,8 +55,13 @@ export class BaseFileSystemRouter extends EventTarget {
   }
 
   async buildRoutes(): Promise<any[]> {
-    for (var src of glob(this.glob())) {
-      await this.addRoute(src);
+    this.initialScan = true;
+    try {
+      for (var src of glob(this.glob())) {
+        await this.addRoute(src);
+      }
+    } finally {
+      this.initialScan = false;
     }
 
     return this.routes;
@@ -119,7 +124,15 @@ export class BaseFileSystemRouter extends EventTarget {
     }
   }
 
+  // "reload" listeners invalidate the routes manifest module, so the events
+  // must stay quiet while buildRoutes first discovers the existing files:
+  // a manifest already served to the browser would otherwise be invalidated
+  // ~200ms into the first request, racing hydration with an HMR update of
+  // the app/router module chain.
+  private initialScan = false;
+
   reload(route: string, type: "update" | "remove" | "add") {
+    if (this.initialScan) return;
     this.dispatchEvent(
       new CustomEvent("reload", {
         detail: {