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/decisions/src/decision_detail_view.dart

@@ -108,15 +108,7 @@ class _DecisionDetailViewState extends State<DecisionDetailView> {
           ),
           if (body != null && body.isNotEmpty) ...[
             const SizedBox(height: 12),
-            Container(
-              padding: const EdgeInsets.all(12),
-              decoration: BoxDecoration(
-                color: tokens.panelBackground,
-                borderRadius: BorderRadius.circular(4),
-                border: Border.all(color: tokens.panelBorder),
-              ),
-              child: ClideText(body, fontSize: 13, fontFamily: clideMonoFamily),
-            ),
+            ClideMarkdown(body),
           ],
           if (refs.isNotEmpty) ...[
             const SizedBox(height: 16),