feat(draw): data-lightbox tap-to-zoom on the drawing card (T-318)

A data-lightbox element gets a tap target (click cursor) over its region;
DrawingCard exposes an onLightbox callback, which the conversation wires to
open the whole drawing in a ClideLightbox (zoomable). Completes the
per-object overlay — captions + lightbox — and T-318's defined scope.
Widget-tested (the tap fires the callback).

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

test/widgets/draw/drawing_card_test.dart

@@ -43,4 +43,21 @@ void main() {
     expect(find.text('Node A'), findsOneWidget);
     expect(find.text('the entry point'), findsOneWidget);
   });
+
+  testWidgets('a data-lightbox element fires onLightbox when tapped (T-318)', (tester) async {
+    var tapped = false;
+    final doc = buildSvgDocument('<svg viewBox="0 0 100 50"><rect x="0" y="0" width="100" height="50" data-lightbox=""/></svg>');
+    await tester.pumpWidget(
+      anchoredHarness(
+        f,
+        SizedBox(
+          width: 400,
+          child: DrawingCard(document: doc, onLightbox: () => tapped = true),
+        ),
+      ),
+    );
+    await tester.pump();
+    await tester.tap(find.byType(DrawingCard));
+    expect(tapped, isTrue);
+  });
 }