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/kernel/src/facade.dart

@@ -345,6 +345,11 @@ class ClideKernel extends InheritedWidget {
     return w.services;
   }
 
+  /// Like [of] but returns null instead of throwing when there is no kernel in
+  /// scope — lets widget-facing facades (e.g. [ClideSettings.i18n]) degrade
+  /// gracefully so a primitive renders in an isolated test without a kernel.
+  static KernelServices? maybeOf(BuildContext context) => context.dependOnInheritedWidgetOfExactType<ClideKernel>()?.services;
+
   @override
   bool updateShouldNotify(ClideKernel oldWidget) => services != oldWidget.services;
 }