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/markdown/src/markdown_viewer.dart

@@ -78,16 +78,12 @@ class _MarkdownViewerState extends State<MarkdownViewer> {
         child: ClideText('Open a .md file to preview it here.', muted: true),
       );
     }
-    final tokens = ClideTheme.of(context).surface;
     return ClidePaneChrome(
       title: _path ?? 'viewer',
       subtitle: '${_content!.split('\n').length} lines',
       child: SingleChildScrollView(
         padding: const EdgeInsets.all(12),
-        child: Text(
-          _content!,
-          style: TextStyle(color: tokens.globalForeground, fontSize: 13, fontFamily: clideMonoFamily, fontFamilyFallback: clideMonoFamilyFallback),
-        ),
+        child: ClideMarkdown(_content!),
       ),
     );
   }