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/i18n/catalog/core_en_us.json

@@ -0,0 +1,27 @@
+{
+  "collapser.expand": { "translation": "Expand" },
+  "collapser.collapse": { "translation": "Collapse" },
+  "collapser.expanded": { "translation": "expanded" },
+  "collapser.collapsed": { "translation": "collapsed" },
+  "toast.dismiss": { "translation": "Dismiss notification" },
+  "lightbox.close": { "translation": "close" },
+  "lightbox.hint": { "translation": "scroll to zoom · double-click to reset · Esc to close" },
+  "tab.new": { "translation": "New tab" },
+  "exline.notCommand": { "translation": "Not an editor command" },
+  "spine.expandSuffix": { "translation": "click to expand" },
+  "pane.close": { "translation": "Close pane" },
+  "pane.header": { "translation": "pane header: {title}" },
+  "reader.back": { "translation": "Back" },
+  "reader.forward": { "translation": "Forward" },
+  "reader.jumpToPin": { "translation": "Jump to pin" },
+  "reader.edit": { "translation": "Edit in editor" },
+  "reader.pin": { "translation": "Pin" },
+  "reader.unpin": { "translation": "Unpin" },
+  "link.openInEditor": { "translation": "Open in editor" },
+  "resize.axis.width": { "translation": "width" },
+  "resize.axis.height": { "translation": "height" },
+  "resize.sidebar": { "translation": "Sidebar {axis}" },
+  "resize.contextPanel": { "translation": "Context panel {axis}" },
+  "resize.slot": { "translation": "{slot} {axis}" },
+  "resize.pixels": { "translation": "{n} pixels" }
+}