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/kernel/src/theme/tokens.dart

@@ -94,6 +94,8 @@ class SurfaceTokens {
     required this.syntaxComment,
     required this.syntaxMethod,
     required this.syntaxPunct,
+    // selection
+    required this.selectionBackground,
     required this.extensionTokens,
   });
 
@@ -186,6 +188,11 @@ class SurfaceTokens {
   final Color syntaxMethod;
   final Color syntaxPunct;
 
+  /// Background color for text selection highlights. Rendered at ~40 % alpha
+  /// (like the terminal's `globalFocus.withAlpha(0x66)`) so the selected text
+  /// remains legible through the tint.
+  final Color selectionBackground;
+
   /// Extension-declared tokens keyed by their dotted path
   /// (e.g. `ext.sqlite.table.background`).
   final Map<String, Color> extensionTokens;
@@ -296,6 +303,9 @@ abstract class TokenKeys {
   static const syntaxMethod = 'syntax.method';
   static const syntaxPunct = 'syntax.punct';
 
+  // selection
+  static const selectionBackground = 'selection.background';
+
   static const all = <String>[
     globalForeground,
     globalBackground,
@@ -363,5 +373,6 @@ abstract class TokenKeys {
     syntaxComment,
     syntaxMethod,
     syntaxPunct,
+    selectionBackground,
   ];
 }