examples: add 6 new over-engineering survivors + platform-native guide (#109)

New examples (examples/):
- modal-dialog: <dialog> vs Radix/react-modal
- url-params: URLSearchParams vs query-string
- number-formatting: Intl.NumberFormat vs numeral
- infinite-scroll: IntersectionObserver vs react-infinite-scroll-component
- deep-clone: structuredClone vs lodash.cloneDeep / JSON hack
- group-by: Object.groupBy vs lodash.groupBy

New doc (docs/platform-native.md):
Comprehensive reference of platform-native solutions across HTML elements,
CSS, Browser APIs, Node.js stdlib, Python stdlib, and database features.
Covers 60+ cases where the platform already has what developers reach for
a package to do.

examples/modal-dialog.md

@@ -0,0 +1,62 @@
+# Modal Dialog
+
+**Task:** "Add a modal dialog for the delete confirmation."
+
+## Without Ponytail
+
+```bash
+npm install @radix-ui/react-dialog
+# or: npm install react-modal
+```
+
+```jsx
+import * as Dialog from "@radix-ui/react-dialog";
+import { useState } from "react";
+
+export function DeleteModal({ onConfirm, onCancel }) {
+  return (
+    <Dialog.Root>
+      <Dialog.Trigger asChild>
+        <button className="btn-danger">Delete</button>
+      </Dialog.Trigger>
+      <Dialog.Portal>
+        <Dialog.Overlay className="dialog-overlay" />
+        <Dialog.Content className="dialog-content">
+          <Dialog.Title>Confirm deletion</Dialog.Title>
+          <Dialog.Description>This action cannot be undone.</Dialog.Description>
+          <div className="dialog-actions">
+            <Dialog.Close asChild>
+              <button onClick={onCancel}>Cancel</button>
+            </Dialog.Close>
+            <button className="btn-danger" onClick={onConfirm}>Delete</button>
+          </div>
+        </Dialog.Content>
+      </Dialog.Portal>
+    </Dialog.Root>
+  );
+}
+```
+
+A dependency, a portal, an overlay, a root, a trigger, a content wrapper — to show a box with two buttons.
+
+## With Ponytail
+
+```html
+<!-- ponytail: browser has one, with focus trapping and backdrop built in -->
+<dialog id="confirm-delete">
+  <p>This action cannot be undone.</p>
+  <button id="cancel">Cancel</button>
+  <button id="confirm">Delete</button>
+</dialog>
+```
+
+```js
+const dialog = document.getElementById("confirm-delete");
+document.getElementById("cancel").onclick = () => dialog.close();
+document.getElementById("confirm").onclick = () => { onConfirm(); dialog.close(); };
+
+// Open it:
+dialog.showModal();
+```
+
+**1 dependency + 30 lines → 0 dependencies + 8 lines.** The native `<dialog>` traps focus automatically, closes on Escape, renders a backdrop via `::backdrop`, and is accessible by default. All browsers since 2022. The library was solving a problem the platform solved.