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>

lib/builtin/claude/src/transcript_reader.dart

@@ -196,7 +196,15 @@ final class ImageMessage extends ConversationItem {
 /// paints (already lowered from the doc's template / primitive source);
 /// [label] / [description] are the optional card caption.
 final class DrawingMessage extends ConversationItem {
-  const DrawingMessage({required super.uuid, required super.timestamp, required super.isSidechain, required this.svg, this.label, this.description});
+  const DrawingMessage({
+    required super.uuid,
+    required super.timestamp,
+    required super.isSidechain,
+    required this.svg,
+    this.label,
+    this.description,
+    this.source,
+  });
 
   /// The SVG document source the renderer paints.
   final String svg;
@@ -204,6 +212,10 @@ final class DrawingMessage extends ConversationItem {
   /// Optional card-level caption (label + supporting description).
   final String? label, description;
 
+  /// Optional template source (e.g. the d2 diagram text) — shown in a collapsed
+  /// "view source" disclosure on the card when present (T-494).
+  final String? source;
+
   @override
   String toString() => 'DrawingMessage(${label ?? '<svg>'})';
 }