Tab row: fit nine tabs inside the centre column

The ninth tab (Shader) pushed the row past the centre column: List
ended at 962 while the column ended at 925, so the last buttons sat
under the inspector and clickText('Container') hit the List tab
instead (CI runs 5 and 6, module 16). Tab buttons now use padding 4
via a new Button padding option; e2e step 14b guards the row width
using the widget box rather than getBounds(), which sees only the
label on an untoggled ghost button.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/editor/debugHook.ts

@@ -181,6 +181,7 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     textOverflows,
     labelsDropped,
     shownLabel,
+    tabBounds: () => editor.tabBounds(),
     viewportCenter: () => centreOf(viewport),
     /** Its drawn box – a step that wants an empty spot must stay inside what the console left over. */
     viewportRect: () => {