test(draw): cover the 2.9.0 drawing-card additions to clear the coverage floor

The drawing-card feature batch (icon/image/compare/graph/d2 cards, --stdin,
tool resolution) added widget + wiring code that dipped total coverage under
the 95% floor — surfaced by `make release` (push-check skips coverage). Cover
the gaps: ProblemsController.refresh, every SVG shape-type bbox + the style
vocabulary, _spawnD2 via a real /bin/cat, the icon-show bus path + error
branches, and quad/arc/close marker paths.

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

test/draw/graph_template_test.dart

@@ -54,6 +54,20 @@ void main() {
     expect((r as DrawErr).message, contains('duplicate'));
   });
 
+  test('an edge from an unknown node is an error', () async {
+    final r = await handler(
+      doc({
+        'nodes': [
+          {'id': 'a'},
+        ],
+        'edges': [
+          {'from': 'ghost', 'to': 'a'},
+        ],
+      }),
+    );
+    expect((r as DrawErr).message, contains('ghost'));
+  });
+
   test('an edge to an unknown node is an error', () async {
     final r = await handler(
       doc({