chore: create error for missing foreign

documentation/docs/98-reference/.generated/client-errors.md

@@ -225,6 +225,12 @@ Cannot do `bind:%key%={undefined}` when `%key%` has a fallback value
 Rest element properties of `$props()` such as `%property%` are readonly
 ```
 
+### renderer_missing_foreign
+
+```
+A custom renderer must define `foreign` to interleave with DOM or another custom renderer
+```
+
 ### rune_outside_svelte
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -171,6 +171,10 @@ This can happen if you render a hydratable on the client that was not rendered o
 
 > Rest element properties of `$props()` such as `%property%` are readonly
 
+## renderer_missing_foreign
+
+> A custom renderer must define `foreign` to interleave with DOM or another custom renderer
+
 ## rune_outside_svelte
 
 > The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files

packages/svelte/src/internal/client/errors.js

@@ -428,6 +428,22 @@ export function props_rest_readonly(property) {
 	}
 }
 
+/**
+ * A custom renderer must define `foreign` to interleave with DOM or another custom renderer
+ * @returns {never}
+ */
+export function renderer_missing_foreign() {
+	if (DEV) {
+		const error = new Error(`renderer_missing_foreign\nA custom renderer must define \`foreign\` to interleave with DOM or another custom renderer\nhttps://svelte.dev/e/renderer_missing_foreign`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/renderer_missing_foreign`);
+	}
+}
+
 /**
  * The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
  * @param {string} rune