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/main.dart

@@ -557,6 +557,9 @@ Future<List<ThemeDefinition>> _loadBundledThemes() async {
 /// catalogs load lazily on activate in later tiers.
 
 const List<String> _tier0Namespaces = [
+  // Framework chrome that lives outside any extension (lib/widgets, lib/kernel,
+  // shared reader chrome) resolves under the 'core' namespace (T-469).
+  'core',
   'builtin.default-layout',
   'builtin.welcome',
   'builtin.ipc-status',