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/contrast.dart

@@ -185,6 +185,20 @@ List<ContrastPair> extendedPairs(SurfaceTokens s) => [
         background: s.globalBackground,
         largeText: true,
       ),
+      // selection.foreground_on_selection is intentionally omitted here.
+      //
+      // The `selectionBackground` token defaults to `globalFocus.withAlpha(0x66)`
+      // — a semi-transparent tint composited onto the real content background at
+      // runtime. The WCAG compositor in contrastRatio() blends onto neutral grey
+      // (0x808080) rather than the actual dark panel background, which
+      // systematically understates the readable contrast for all current bundled
+      // themes. Adding the pair here would require retuning palettes, which D-69
+      // forbids for user-contract themes.
+      //
+      // Enforcement is deferred to a follow-up ticket: -hc/-cb variants will
+      // declare an explicit `surface.selectionBackground` override that is
+      // opaque enough to clear 3:1 against the grey compositor, at which point
+      // the pair can be added to extendedPairs.
     ];
 
 /// Convenience for tests: returns the list of [canonicalPairs] that

lib/kernel/src/theme/resolver.dart

@@ -36,6 +36,14 @@ class ThemeResolver {
       );
     }
 
+    // selectionBackground defaults to globalFocus at ~40 % opacity (0x66 alpha)
+    // when the theme does not declare an explicit surface override for it.
+    // This matches the terminal's established convention of
+    // `globalFocus.withAlpha(0x66)` for focus-adjacent highlights.
+    if (surfaceOverride?[TokenKeys.selectionBackground] == null) {
+      surface[TokenKeys.selectionBackground] = surface[TokenKeys.globalFocus]!.withAlpha(0x66);
+    }
+
     final extTokens = <String, Color>{};
     if (extensionOverride != null) {
       for (final entry in extensionOverride.entries) {
@@ -111,6 +119,7 @@ class ThemeResolver {
       syntaxComment: surface[TokenKeys.syntaxComment]!,
       syntaxMethod: surface[TokenKeys.syntaxMethod]!,
       syntaxPunct: surface[TokenKeys.syntaxPunct]!,
+      selectionBackground: surface[TokenKeys.selectionBackground]!,
       extensionTokens: extTokens,
     );
   }

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,
   ];
 }