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/widgets/widgets.dart

@@ -7,8 +7,11 @@ library;
 
 export 'src/clide_button.dart';
 export 'src/clide_column_hat.dart';
+export 'src/clide_code_block.dart';
 export 'src/clide_divider.dart';
 export 'src/clide_filter_box.dart';
+export 'src/clide_markdown.dart';
+export 'src/clide_svg_view.dart';
 export 'src/clide_icon.dart';
 export 'src/clide_icon_rail.dart';
 export 'src/clide_palette.dart';