chore: add comment

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/utils.js

@@ -370,6 +370,9 @@ export function validate_mutation(node, context, expression) {
 export function build_expression(context, expression, metadata, state = context.state) {
 	const value = /** @type {Expression} */ (context.visit(expression, state));
 
+	// Components not explicitly in legacy mode might be expected to be in runes mode (especially since we didn't
+	// adjust this behavior until recently, which broke people's existing components), so we also bail in this case.
+	// Kind of an in-between-mode.
 	if (context.state.analysis.runes || context.state.analysis.maybe_runes) {
 		return value;
 	}