add ClideMarkdown, ClideCodeBlock, ClideSvgView primitives

Three reusable rendering widgets in lib/widgets/:

- ClideMarkdown: walks markdown AST (GFM tables, task lists,
  fenced code), renders headings, lists, blockquotes, tables,
  inline bold/italic/code/links. Delegates code blocks to
  ClideCodeBlock.

- ClideCodeBlock: syntax-highlighted code via TreeSitterService.
  Async highlight, byte-to-char offset mapping, theme-aware
  colors from SurfaceTokens.

- ClideSvgView: wraps jovial_svg with asset/string constructors
  and optional sizing.

Retrofitted: decision detail body renders as markdown instead of
raw text. Markdown viewer uses ClideMarkdown. Welcome logo uses
ClideSvgView.

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

lib/builtin/welcome/src/welcome_view.dart

@@ -2,9 +2,7 @@ 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});
@@ -56,13 +54,7 @@ class _Header extends StatelessWidget {
     return Row(
       crossAxisAlignment: CrossAxisAlignment.center,
       children: [
-        SizedBox(
-          width: 144,
-          height: 144,
-          child: ScalableImageWidget.fromSISource(
-            si: ScalableImageSource.fromSvg(rootBundle, 'assets/logo/logo.svg'),
-          ),
-        ),
+        const ClideSvgView.asset('assets/logo/logo.svg', width: 144, height: 144),
         const SizedBox(width: 24),
         Column(
           crossAxisAlignment: CrossAxisAlignment.start,