fix(angular-virtual): capture _didMount cleanup to remove scroll/resize listeners on destroy

afterNextRender discarded the teardown function returned by _didMount(), so observeWindowOffset's scroll and resize listeners were never unregistered.
Post-destroy onChange callbacks kept mutating virtualizerSignal, corrupting Angular's reactive graph and causing a "Cannot read properties of null (reading 'flags')" crash on next component creation.

Also fixes the cleanup variable type annotation.

Author: jsaguet

.changeset/fix-angular-virtual-cleanup.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/angular-virtual': patch
+---
+
+fix: capture _didMount cleanup to remove scroll/resize listeners on destroy

packages/angular-virtual/src/index.ts

@@ -70,8 +70,12 @@ function createVirtualizerBase<
     { allowSignalWrites: true },
   )
 
-  let cleanup: () => void | undefined
-  afterNextRender({ read: () => (virtualizer ?? lazyInit())._didMount() })
+  let cleanup: (() => void) | undefined
+  afterNextRender({
+    read: () => {
+      cleanup = (virtualizer ?? lazyInit())._didMount()
+    },
+  })
 
   inject(DestroyRef).onDestroy(() => cleanup?.())