docs: use a canvas demo for $effect

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -196,48 +196,58 @@ In essence, `$derived(expression)` is equivalent to `$derived.by(() => expressio
 
 ## `$effect`
 
-To run side-effects like logging or analytics whenever some specific values change, or when a component is mounted to the DOM, we can use the `$effect` rune:
+To run _side-effects_ when the component is mounted to the DOM, and when values change, we can use the `$effect` rune ([demo](/#H4sIAAAAAAAAE31T24rbMBD9lUG7kAQ2sbdlX7xOYNk_aB_rQhRpbAsU2UiTW0P-vbrYubSlYGzmzMzROTPymdVKo2PFjzMzfIusYB99z14YnfoQuD1qQh-7bmdFQEonrOppVZmKNBI49QthCc-OOOH0LZ-9jxnR6c7eUpOnuv6KeT5JFdcqbvbcBcgDz1jXKGg6ncFyBedYR6IzLrAZwiN5vtSxaJA-EzadfJEjKw11C6GR22-BLH8B_wxdByWpvUYtqqal2XB6RVkG1CoHB6U1WJzbnYFDiwb3aGEdDa3Bm1oH12sQLTcNPp7r56m_00mHocSG97_zd7ICUXonA5fwKbPbkE2ZtMJGGVkEdctzQi4QzSwr9prnFYNk5hpmqVuqPQjNnfOJoMF22lUsrq_UfIN6lfSVyvQ7grB3X2mjMZYO3XO9w-U5iLx42qg29md3BP_ni5P4gy9ikTBlHxjLzAtPDlyYZmRdjAbGq7HprEQ7p64v4LU_guu0kvAkhBim3nMplWl8FreQD-CW20aZR0wq12t-KqDWeBywhvexKC3memmDwlHAv9q4Vo2ZK8KtK0CgX7u9J8wXbzdKv-nRnfF_2baTqlYoWUF2h5efl9-n0O6koAMAAA==)):
 
 ```svelte
 <script>
-	let count = $state(0);
-	let doubled = $derived(count * 2);
+	let size = $state(50);
+	let color = $state('#ff3e00');
+
+	let canvas;
 
 	$effect(() => {
-		console.log({ count, doubled });
+		const context = canvas.getContext('2d');
+		context.clearRect(0, 0, canvas.width, canvas.height);
+
+		// this will re-run whenever `color` or `size` change
+		context.fillStyle = color;
+		context.fillRect(0, 0, size, size);
 	});
 </script>
 
-<button on:click={() => count++}>
-	{doubled}
-</button>
-
-<p>{count} doubled is {doubled}</p>
+<canvas bind:this={canvas} width="100" height="100" />
 ```
 
-`$effect` will automatically subscribe to any `$state` or `$derived` values it reads _synchronously_ and reruns whenever their values change — that means, values after an `await` or inside a `setTimeout` will _not_ be tracked. `$effect` will run after the DOM has been updated.
+The function passed to `$effect` will run when the component mounts, and will re-run after any changes to the values it reads that were declared with `$state` or `$derived` (including those passed in with `$props`). Re-runs are batched (i.e. changing `color` and `size` in the same moment won't cause two separate runs), and happen after any DOM updates have been applied.
 
-```svelte
-<script>
-	let count = $state(0);
-	let doubled = $derived(count * 2);
+Values that are read asynchronously — after an `await` or inside a `setTimeout`, for example — will _not_ be tracked. Here, the canvas will be repainted when `color` changes, but not when `size` changes ([demo](/#H4sIAAAAAAAAE31T24rbMBD9lUG7kCxsbG_LvrhOoPQP2r7VhSjy2BbIspHGuTT436tLnMtSCiaOzpw5M2dGPrNaKrQs_3VmmnfIcvZ1GNgro9PgD3aPitCdbT8a4ZHCCiMH2pS6JIUEVv5BWMOzJU64fM9evswR0ave3EKLp7r-jFm2iIwri-s9tx5ywDPWNQpaLl9gvYFz4JHotfVqmvBITi9mJA3St4gtF5-qWZUuvEQo5Oa7F8tewT2XrIOsqL2eWpRNS7eGSkpToFZaOEilwODKjBoOLWrco4FtsLQF0XLdoE2S5LGmm6X6QSflBxKod8IW6afssB8_uAslndJuJNA9hWKw9VO91pmJ92XunHlu_J1nMDk8_p_8q0hvO9NFtA47qavcW12fIzJBmM26ZG9ZVjKIs7ke05hdyT0Ixa11Ad-P6ZUtWbgNheI7VJvYQiH14Bz5a-SYxvtwIqHonqsR12ff8ORkQ-chP70T-L9eGO4HvYAFwRh9UCxS13h0YP2CgmoyG5h3setNhWZF_ZDD23AE2ytZwZMQ4jLYgVeV1I2LYgfZBey4aaR-xCppB8VPOdQKjxes4UMgxcVcvwHf4dzAv9K4ko1eScLO5iDQXQFzL5gl7zdJt-nZnXYfbddXspZYsZzMiNPv6S8Bl41G7wMAAA==)):
 
-	$effect(() => {
-		// runs after the DOM has been updated
-		// when the component is mounted
-		// and whenever `count` changes,
-		// but not when `doubled` changes,
-		console.log(count);
-
-		setTimeout(() => console.log(doubled));
-	});
-</script>
-
-<button on:click={() => count++}>
-	{doubled}
-</button>
+```ts
+// @filename: index.ts
+declare let canvas: {
+	width: number;
+	height: number;
+	getContext(
+		type: '2d',
+		options?: CanvasRenderingContext2DSettings
+	): CanvasRenderingContext2D;
+};
+declare let color: string;
+declare let size: number;
 
-<p>{count} doubled is {doubled}</p>
+// ---cut---
+$effect(() => {
+	const context = canvas.getContext('2d');
+	context.clearRect(0, 0, canvas.width, canvas.height);
+
+	// this will re-run whenever `color` changes...
+	context.fillStyle = color;
+
+	setTimeout(() => {
+		// ...but not when `size` changes
+		context.fillRect(0, 0, size, size);
+	}, 0);
+});
 ```
 
 An effect only reruns when the object it reads changes, not when a property inside it changes. If you want to react to _any_ change inside an object for inspection purposes at dev time, you may want to use [`inspect`](#$inspect).