add format engine packages: jovial_svg, markdown, html_core (D-058)

Three format engines adopted per the new D-058 rule (format
engines clear the dependency bar; UI chrome stays in-house):

- jovial_svg 1.1.26 (BSD-3) — SVG renderer via CustomPaint
- markdown 7.2.2 (BSD-3) — GFM parser; renderer is ours
- flutter_widget_from_html_core 0.17.2 (MIT) — HTML renderer

Welcome screen now loads logo.svg via jovial_svg instead of a
raster PNG. POLICY.md updated with format engine rule. All three
documented in licenses.yaml.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/welcome/src/welcome_view.dart

@@ -2,7 +2,9 @@ import 'dart:async';
 
 import 'package:clide/kernel/kernel.dart';
 import 'package:clide/widgets/widgets.dart';
+import 'package:flutter/services.dart' show rootBundle;
 import 'package:flutter/widgets.dart';
+import 'package:jovial_svg/jovial_svg.dart';
 
 class WelcomeView extends StatelessWidget {
   const WelcomeView({super.key});
@@ -54,7 +56,13 @@ class _Header extends StatelessWidget {
     return Row(
       crossAxisAlignment: CrossAxisAlignment.center,
       children: [
-        Image.asset('assets/logo/logo-192.png', width: 72, height: 72),
+        SizedBox(
+          width: 72,
+          height: 72,
+          child: ScalableImageWidget.fromSISource(
+            si: ScalableImageSource.fromSvg(rootBundle, 'assets/logo/logo.svg'),
+          ),
+        ),
         const SizedBox(width: 24),
         Column(
           crossAxisAlignment: CrossAxisAlignment.start,