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