add section on inspecting state

Author: Rich-Harris

apps/svelte.dev/content/tutorial/01-svelte/02-reactivity/04-inspecting-state/+assets/app-a/src/lib/App.svelte

@@ -0,0 +1,15 @@
+<script>
+	let numbers = $state([1, 2, 3, 4]);
+	let total = $derived(numbers.reduce((t, n) => t + n, 0));
+
+	function addNumber() {
+		numbers.push(numbers.length + 1);
+		console.log(numbers);
+	}
+</script>
+
+<p>{numbers.join(' + ')} = {total}</p>
+
+<button onclick={addNumber}>
+	Add a number
+</button>

apps/svelte.dev/content/tutorial/01-svelte/02-reactivity/04-inspecting-state/+assets/app-b/src/lib/App.svelte

@@ -0,0 +1,16 @@
+<script>
+	let numbers = $state([1, 2, 3, 4]);
+	let total = $derived(numbers.reduce((t, n) => t + n, 0));
+
+	function addNumber() {
+		numbers.push(numbers.length + 1);
+	}
+
+	$inspect(numbers).with(console.trace);
+</script>
+
+<p>{numbers.join(' + ')} = {total}</p>
+
+<button onclick={addNumber}>
+	Add a number
+</button>

apps/svelte.dev/content/tutorial/01-svelte/02-reactivity/04-inspecting-state/index.md

@@ -0,0 +1,36 @@
+---
+title: Inspecting state
+---
+
+It's often useful to be able to track the value of a piece of state as it changes over time.
+
+Inside the `addNumber` function, we've added a `console.log` statement. But if you click the button and open the console drawer (using the button to the right of the URL bar), you'll see a warning, and a message saying the message could not be cloned.
+
+That's because `numbers` is a reactive [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy). There's a couple of things we can do. Firstly, we can create a non-reactive _snapshot_ of the state with `$state.snapshot(...)`:
+
+```js
+/// file: App.svelte
+function addNumber() {
+	numbers.push(numbers.length + 1);
+	console.log(+++$state.snapshot(numbers)+++);
+}
+```
+
+Alternatively, we can use the `$inspect` rune to automatically log a snapshot of the state whenever it changes. This code will automatically be stripped out of your production build:
+
+```js
+/// file: App.svelte
+function addNumber() {
+	numbers.push(numbers.length + 1);
+	---console.log($state.snapshot(numbers));---
+}
+
++++$inspect(numbers);+++
+```
+
+You can customise how the information is displayed by using `$inspect(...).with(fn)` — for example, you can use `console.trace` to see where the state change originated from:
+
+```js
+/// file: App.svelte
+$inspect(numbers)+++.with(console.trace)+++;
+```