docs: document $state.snapshot toJSON behavior (#18154)

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`$state.snapshot` already clones the value returned from `toJSON`, and
its `Snapshot<T>` type reflects that return type. The `$state.snapshot`
docs now call out that behavior explicitly, including the generated
ambient types shown by editors.

Test plan: `pnpm check`; `pnpm lint`.

Fixes #18129

Co-authored-by: sakaenyeniceri5 <sakaenyeniceri5@users.noreply.github.com>

documentation/docs/02-runes/02-$state.md

@@ -167,6 +167,8 @@ To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snaps
 
 This is handy when you want to pass some state to an external library or API that doesn't expect a proxy, such as `structuredClone`.
 
+If a value has a `toJSON` method, the snapshot will clone the value returned from `toJSON` instead of the original object.
+
 ## `$state.eager`
 
 When state changes, it may not be reflected in the UI immediately if it is used by an `await` expression, because [updates are synchronized](await-expressions#Synchronized-updates).

packages/svelte/src/ambient.d.ts

@@ -147,6 +147,8 @@ declare namespace $state {
 	 * </script>
 	 * ```
 	 *
+	 * If `state` has a `toJSON` method, the snapshot will clone the value returned from `toJSON` instead of the original object.
+	 *
 	 * @see {@link https://svelte.dev/docs/svelte/$state#$state.snapshot Documentation}
 	 *
 	 * @param state The value to snapshot

packages/svelte/types/index.d.ts

@@ -3345,6 +3345,8 @@ declare namespace $state {
 	 * </script>
 	 * ```
 	 *
+	 * If `state` has a `toJSON` method, the snapshot will clone the value returned from `toJSON` instead of the original object.
+	 *
 	 * @see {@link https://svelte.dev/docs/svelte/$state#$state.snapshot Documentation}
 	 *
 	 * @param state The value to snapshot