Tab row: what does not fit goes behind a menu, the active tab always stays (#84)

Nine tabs need 631 px; at 1280x800 the centre column offers 666 px, so the tenth
tab would have spilled under the inspector - the same overflow CI caught in run 5.

TabStrip replaces the bare HStack: it fits the longest prefix that stays inside
the budget, always appends the active tab, and offers the rest behind a fixed
28 px button. The button's width is fixed on purpose - deriving it from the
number of hidden tabs would make width depend on the count and the count on the
width.

The rule itself is a pure function (tabRow.ts), so it is checked without a
browser. setActive() calls relayout() itself because Widget.resize only lays out
on a changed size, and switching tabs does not change any size.

Adding a tab now needs no space arithmetic - only a line in TAB_ORDER,
TAB_LABELS and panes, all three Record<CentreTab, ...>, so forgetting one is a
type error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/editor/debugHook.ts

@@ -186,7 +186,9 @@ export function installDebugHook(app: Application, state: EditorState, editor: E
     textOverflows,
     labelsDropped,
     shownLabel,
-    tabBounds: () => editor.tabBounds(),
+    tabRow: () => editor.tabRow(),
+    /** The dividers left and right of the centre column – dragging them narrows the tab row. */
+    panelSplitters: () => ({ left: centreOf(editor.leftSplitter), right: centreOf(editor.rightSplitter) }),
     viewportCenter: () => centreOf(viewport),
     /** Pans the Scene tab to a world point – a step's own lane, away from what earlier steps left. */
     lookAt: (x: number, y: number) => viewport.lookAt(x, y),