refactor(i18n): route framework + shared chrome through a 'core' catalog (T-469)

Framework strings outside any extension — widget primitives (collapser, toast,
lightbox, multitab, ex-line, spine, pane chrome), the shared reader chrome, the
markdown 'Open in editor' tooltip, and the drag-resize handle a11y labels — now
resolve under a new 'core' namespace (preloaded at boot). Settles the T-469
namespace question: framework chrome gets one 'core' catalog.

Makes ClideSettings.i18n.string/.interpolated null-safe (ClideKernel.maybeOf):
primitives render kernel-less in isolated tests, returning the placeholder —
matching the D-101 fallback contract for fonts. The markdown tooltip threads
via the ClideMarkdownHooks carrier like mono/ui; drag_resize reads the kernel
i18n directly to avoid a kernel→widgets layering inversion. No en_US change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/src/clide_spine.dart

@@ -19,9 +19,10 @@ class ClideSpine extends StatelessWidget {
     final tokens = ClideSettings.theme.of(context).surface;
     final borderSide = BorderSide(color: tokens.dividerColor);
 
+    final expandSuffix = ClideSettings.i18n.string(context, 'spine.expandSuffix', namespace: 'core', placeholder: 'click to expand');
     return Semantics(
       button: true,
-      label: '$label — click to expand',
+      label: '$label — $expandSuffix',
       child: ClideTappable(
         onTap: onExpand,
         builder: (context, hovered, _) => Container(