feat(draw): fold the d2 source into a "view source" disclosure (T-494)

The rendered diagram leads; the d2 source carries through the draw bus and
folds into a collapsed ClideCollapserCard beneath the card (D-78 display-
only). en+nl catalogs. Also switches the d2 test to a null-aware element.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/draw/d2_template_test.dart

@@ -4,7 +4,7 @@ import 'package:clide/src/draw/draw_doc.dart';
 import 'package:test/test.dart';
 
 void main() {
-  DrawingCardDoc d2doc(Object? source) => DrawingCardDoc(template: 'd2', fields: {'template': 'd2', if (source != null) 'source': source});
+  DrawingCardDoc d2doc(Object? source) => DrawingCardDoc(template: 'd2', fields: {'template': 'd2', 'source': ?source});
 
   group('d2TemplateHandler', () {
     test('compiles the source field via the injected compiler', () async {