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_toast.dart

@@ -86,7 +86,7 @@ class _ClideToastState extends State<ClideToast> {
                   const SizedBox(width: clideGapStandard),
                   Semantics(
                     button: true,
-                    label: 'Dismiss notification',
+                    label: ClideSettings.i18n.string(context, 'toast.dismiss', namespace: 'core', placeholder: 'Dismiss notification'),
                     child: ClideTappable(
                       onTap: widget.onDismiss,
                       builder: (ctx, hovered, _) =>