Fix change tracking for array items accessed via iteration methods (find, forEach, for...of, etc.)

.changeset/fix-array-iteration-proxy.md

@@ -0,0 +1,20 @@
+---
+"@tanstack/db": patch
+---
+
+Fix change tracking for array items accessed via iteration methods (find, forEach, for...of, etc.)
+
+Previously, modifications to array items retrieved via iteration methods were not tracked by the change proxy because these methods returned raw array elements instead of proxied versions. This caused `getChanges()` to return an empty object, which in turn caused `createOptimisticAction`'s `mutationFn` to never be called when using patterns like:
+
+```ts
+collection.update(id, (draft) => {
+  const item = draft.items.find((x) => x.id === targetId)
+  if (item) {
+    item.value = newValue // This change was not tracked!
+  }
+})
+```
+
+The fix adds proxy handling for array iteration methods similar to how Map/Set iteration is already handled, ensuring that callbacks receive proxied elements and returned elements are properly proxied.
+
+Also refactors proxy.ts for improved readability by extracting helper functions and hoisting constants to module scope.