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/ex_line_overlay.dart

@@ -181,7 +181,11 @@ class _ExLineOverlayState extends State<ExLineOverlay> {
                     if (_rejected)
                       Padding(
                         padding: const EdgeInsets.symmetric(horizontal: 12, vertical: 6),
-                        child: ClideText('Not an editor command', fontSize: clideFontCaption, color: tokens.statusError),
+                        child: ClideText(
+                          ClideSettings.i18n.string(context, 'exline.notCommand', namespace: 'core', placeholder: 'Not an editor command'),
+                          fontSize: clideFontCaption,
+                          color: tokens.statusError,
+                        ),
                       ),
                   ],
                 ),