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/daemon/draw_commands_test.dart

@@ -48,7 +48,7 @@ void main() {
 
   test('template doc lowers via the registry before publishing', () async {
     wire();
-    registry.register('d2', (doc) async => '<svg data-d2="${doc.fields['source']}"/>');
+    registry.register('d2', (doc) async => DrawOk('<svg data-d2="${doc.fields['source']}"/>'));
     files['c.json'] = '{"template":"d2","source":"a -> b"}';
     final r = await draw('c.json');
     expect(r.ok, isTrue, reason: r.error?.message);
@@ -56,6 +56,24 @@ void main() {
     expect(r.data['template'], 'd2');
   });
 
+  test('a .d2 file is wrapped as a d2 template and lowered (T-494)', () async {
+    wire();
+    registry.register('d2', (doc) async => DrawOk('<svg data-d2="${doc.fields['source']}"/>'));
+    files['pipeline.d2'] = 'a -> b';
+    final r = await draw('pipeline.d2');
+    expect(r.ok, isTrue, reason: r.error?.message);
+    expect(published.single.data['svg'], '<svg data-d2="a -> b"/>');
+    expect(r.data['template'], 'd2');
+  });
+
+  test('a .svg file renders as a primitive card (T-494)', () async {
+    wire();
+    files['logo.svg'] = '<svg id="raw"/>';
+    final r = await draw('logo.svg');
+    expect(r.ok, isTrue, reason: r.error?.message);
+    expect(published.single.data['svg'], '<svg id="raw"/>');
+  });
+
   test('a missing file → notFound, nothing published', () async {
     wire();
     final r = await draw('nope.json');