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/group-by.md

@@ -0,0 +1,35 @@
+# Group By
+
+**Task:** "Group this array of objects by a key."
+
+## Without Ponytail
+
+```bash
+npm install lodash
+```
+
+```js
+import { groupBy } from "lodash";
+
+const byStatus = groupBy(orders, "status");
+// → { pending: [...], shipped: [...], delivered: [...] }
+```
+
+Or the hand-rolled version:
+
+```js
+const byStatus = orders.reduce((acc, order) => {
+  (acc[order.status] ??= []).push(order);
+  return acc;
+}, {});
+```
+
+## With Ponytail
+
+```js
+// ponytail: Object.groupBy does this
+const byStatus = Object.groupBy(orders, order => order.status);
+// → { pending: [...], shipped: [...], delivered: [...] }
+```
+
+**1 dependency (or a reduce) → 1 built-in.** `Object.groupBy` shipped in Chrome 117, Firefox 119, Safari 17.4, Node.js 21. If you need a `Map` instead of a plain object: `Map.groupBy(orders, o => o.status)`. Check your target runtime; if you need IE11 or old Node, the `reduce` one-liner is still the right call — not lodash.