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/multitab_pane.dart

@@ -346,7 +346,7 @@ class _AddButton extends StatelessWidget {
     final tokens = ClideSettings.theme.of(context).surface;
     return Semantics(
       button: true,
-      label: 'New tab',
+      label: ClideSettings.i18n.string(context, 'tab.new', namespace: 'core', placeholder: 'New tab'),
       excludeSemantics: true,
       child: ClideTappable(
         onTap: onTap,