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>

lib/main.dart

@@ -41,6 +41,7 @@ import 'package:clide/clide.dart' show clideVersion;
 import 'package:clide/src/daemon/claude_account_commands.dart';
 import 'package:clide/src/daemon/dispatcher.dart';
 import 'package:clide/src/daemon/draw_commands.dart';
+import 'package:clide/src/draw/d2_template.dart' show d2TemplateHandler;
 import 'package:clide/src/daemon/editor_commands.dart';
 import 'package:clide/src/daemon/files_commands.dart';
 import 'package:clide/src/daemon/git_commands.dart';
@@ -376,7 +377,7 @@ Future<void> main() async {
     registerDrawCommands(
       dispatcher,
       () => kernelMessages?.publish,
-      registry: DrawingRegistry(),
+      registry: DrawingRegistry()..register('d2', d2TemplateHandler()),
       readFile: (path) async {
         final file = File(path.startsWith('/') ? path : '${workRoot.path}/$path');
         try {