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>

README.md

@@ -37,7 +37,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 785 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 790 checks (GM_STEPS=20 for one step)
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs
 node tools/seed-actor.mjs && node tools/smoke-actor.mjs