feat(draw): d2 diagram template — compile d2 source to SVG (T-494)

The d2 drawing template compiles a diagram's source to SVG through the d2
binary (resolved via the D-104 path layer), then paints it with the same
renderer the svg card uses. `clide draw --file x.d2` infers the type from
the extension; `.svg` files render directly. Template handlers now return
a DrawResult so a compile failure or an unresolved d2 surface as an honest
userError with an install hint, not a generic "no SVG". Real d2 0.7.1
verified end to end.

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

test/draw/d2_template_test.dart

@@ -0,0 +1,59 @@
+import 'package:clide/src/draw/d2_template.dart';
+import 'package:clide/src/draw/draw_dispatch.dart';
+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});
+
+  group('d2TemplateHandler', () {
+    test('compiles the source field via the injected compiler', () async {
+      final h = d2TemplateHandler(compile: (s) async => DrawOk('<svg data-s="$s"/>'));
+      final r = await h(d2doc('a -> b'));
+      expect((r as DrawOk).svg, '<svg data-s="a -> b"/>');
+    });
+
+    test('a missing source is an honest error', () async {
+      final h = d2TemplateHandler(compile: (_) async => const DrawOk('x'));
+      expect(await h(d2doc(null)), isA<DrawErr>());
+    });
+
+    test('a blank source is an honest error', () async {
+      final h = d2TemplateHandler(compile: (_) async => const DrawOk('x'));
+      expect(await h(d2doc('   ')), isA<DrawErr>());
+    });
+
+    test('propagates a compiler error message', () async {
+      final h = d2TemplateHandler(compile: (_) async => const DrawErr('boom'));
+      expect((await h(d2doc('a -> b')) as DrawErr).message, 'boom');
+    });
+  });
+
+  group('d2CompileViaBinary', () {
+    Future<DrawResult> compile({String? bin = '/x/d2', D2Run? run}) =>
+        d2CompileViaBinary('a -> b', resolveD2: () => bin, run: run ?? (exe, src) async => (code: 0, out: '<svg/>', err: 'success:'));
+
+    test('unresolved d2 hints how to install', () async {
+      expect((await compile(bin: null) as DrawErr).message, contains('d2 not found'));
+    });
+
+    test('a clean compile returns the SVG (stderr ignored)', () async {
+      expect((await compile() as DrawOk).svg, '<svg/>');
+    });
+
+    test('a non-zero exit is a compile error carrying stderr', () async {
+      final r = await compile(run: (exe, src) async => (code: 1, out: '', err: 'line 2: syntax error'));
+      expect((r as DrawErr).message, contains('syntax error'));
+    });
+
+    test('empty output is an error', () async {
+      final r = await compile(run: (exe, src) async => (code: 0, out: '  ', err: ''));
+      expect(r, isA<DrawErr>());
+    });
+
+    test('a spawn failure is reported honestly', () async {
+      final r = await compile(run: (exe, src) async => throw 'ENOENT');
+      expect((r as DrawErr).message, contains('could not run d2'));
+    });
+  });
+}

test/draw/draw_dispatch_test.dart

@@ -37,15 +37,15 @@ void main() {
     });
 
     test('template: a registered handler lowers the doc to SVG', () async {
-      final reg = DrawingRegistry()..register('d2', (doc) async => '<svg data-src="${doc.fields['source']}"/>');
+      final reg = DrawingRegistry()..register('d2', (doc) async => DrawOk('<svg data-src="${doc.fields['source']}"/>'));
       final r = await resolve(parseDrawingCardDoc({'template': 'd2', 'source': 'a -> b'})!, reg);
       expect((r as DrawOk).svg, '<svg data-src="a -> b"/>');
     });
 
-    test('template: a handler that returns null is an error', () async {
-      final reg = DrawingRegistry()..register('d2', (_) async => null);
+    test('template: a handler error propagates with its message', () async {
+      final reg = DrawingRegistry()..register('d2', (_) async => const DrawErr('d2 not found'));
       final r = await resolve(parseDrawingCardDoc({'template': 'd2'})!, reg);
-      expect(r, isA<DrawErr>());
+      expect((r as DrawErr).message, contains('d2 not found'));
     });
   });
 }