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/deep-clone.md

@@ -0,0 +1,31 @@
+# Deep Clone
+
+**Task:** "Deep clone this object."
+
+## Without Ponytail
+
+```bash
+npm install lodash
+```
+
+```js
+import { cloneDeep } from "lodash";
+
+const copy = cloneDeep(original);
+```
+
+Or the classic hack:
+
+```js
+// fragile: loses Date, undefined, Map, Set, circular refs, functions
+const copy = JSON.parse(JSON.stringify(original));
+```
+
+## With Ponytail
+
+```js
+// ponytail: structuredClone does this
+const copy = structuredClone(original);
+```
+
+**1 dependency (or a fragile hack) → 1 built-in.** `structuredClone` handles `Date`, `Map`, `Set`, `ArrayBuffer`, `RegExp`, circular references, and more — everything `JSON.parse/stringify` silently drops. Available in every browser since 2022 and Node.js since v17. Pull lodash in when you need the rest of it, not for one function.