feat(draw): per-object data-* caption overlay on the drawing card (T-318)

DrawingCard stacks a caption layer over the SVG: each data-label /
data-description annotation renders a themed caption just below its
element, mapped to pixel space via a shared svgViewportFit (refactored out
of the painter so the overlay and the paint use identical viewBox-fit
math). Display-only (IgnorePointer). Widget-tested. data-lightbox is
captured in the model; the tap-to-zoom interaction is the last follow-on.

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

test/widgets/draw/drawing_card_test.dart

@@ -33,4 +33,14 @@ void main() {
     expect(find.byType(SvgView), findsOneWidget);
     expect(find.text('Build pipeline'), findsNothing);
   });
+
+  testWidgets('renders per-object captions from data-* annotations (T-318)', (tester) async {
+    final doc = buildSvgDocument(
+      '<svg viewBox="0 0 100 50"><rect x="10" y="10" width="40" height="20" data-label="Node A" data-description="the entry point"/></svg>',
+    );
+    await tester.pumpWidget(anchoredHarness(f, SizedBox(width: 400, child: DrawingCard(document: doc))));
+    await tester.pump();
+    expect(find.text('Node A'), findsOneWidget);
+    expect(find.text('the entry point'), findsOneWidget);
+  });
 }