make native text widgets selection-aware (T-135)

Foundation for rendering the Claude conversation natively (T-132) with
the cross-widget select+copy the terminal gives today. Converts the raw
RichText in clide_markdown + clide_code_block to Text.rich, which
registers with a Flutter SelectionArea's selection machinery (raw
RichText does not). Adds a selectionBackground surface token
(globalFocus at ~40% alpha, matching the terminal's selection tint) via
tokens + resolver default; bundled palettes are untouched (D-69).

Text and code blocks now select across each other under a SelectionArea;
tables and tappable link-spans remain non-selectable islands for now.
The selection contrast pair is intentionally not added to the WCAG gate:
the tint is semi-transparent and the gate's neutral-grey compositor would
false-fail it (documented in contrast.dart); deferred to the -hc/-cb pass.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/widgets/src/clide_code_block.dart

@@ -76,7 +76,7 @@ class _ClideCodeBlockState extends State<ClideCodeBlock> {
       ),
       child: SingleChildScrollView(
         scrollDirection: Axis.horizontal,
-        child: RichText(text: textSpan),
+        child: Text.rich(textSpan),
       ),
     );
   }