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/number-formatting.md

@@ -0,0 +1,37 @@
+# Number Formatting
+
+**Task:** "Format numbers as currency and with thousand separators."
+
+## Without Ponytail
+
+```bash
+npm install numeral
+# or: npm install accounting
+```
+
+```js
+import numeral from "numeral";
+
+numeral(1234567.89).format("$1,234.00"); // "$1,234,567.89"
+numeral(0.745).format("0.0%");           // "74.5%"
+numeral(1500).format("0.0a");            // "1.5k"
+```
+
+## With Ponytail
+
+```js
+// ponytail: Intl.NumberFormat does this, locale-aware
+new Intl.NumberFormat("en-US", { style: "currency", currency: "USD" })
+  .format(1234567.89);
+// → "$1,234,567.89"
+
+new Intl.NumberFormat("en-US", { style: "percent" })
+  .format(0.745);
+// → "74.5%"
+
+new Intl.NumberFormat("en-US", { notation: "compact" })
+  .format(1500);
+// → "1.5K"
+```
+
+**1 dependency → 0 dependencies.** `Intl.NumberFormat` is built into every JS runtime, handles every locale correctly, and gets currency symbols, decimal separators, and grouping right for any market without a lookup table. A library that hardcodes formats will always be wrong for someone.