Add a changelog for the audit fixes and update the README

CHANGELOG.md

@@ -0,0 +1,97 @@
+# Changelog
+
+## 2026-10-08 to 2026-10-09 — audit fixes
+
+An audit of the site covered bugs, UI and UX, accessibility, performance and
+the teaching content in both languages. Each fix below is its own pull request,
+and all are merged into `main`.
+
+### Bugs
+
+- Turning on predict mode stops autoplay, and focus and screen-reader
+  announcements follow the question flow ([#11]).
+- Keyboard users can mark problems as done; the checkbox has a larger target
+  and long titles show two lines ([#12]).
+- Tabs no longer overwrite each other's progress ([#13]).
+- After React gives up hydrating the root, theme, language and sidebar
+  settings come back from storage ([#36]).
+
+### Teaching content
+
+- Heap: the JavaScript solutions call methods `MinHeap` really has, and the
+  notes cover Python 3.14's max-heap functions ([#14]).
+- Union-find: LC 685, 839 and 947 are argued correctly, and the lab narrates
+  equal-rank unions accurately ([#15]).
+- Atlas: final quiz question 6, the complexity table and the decision tree
+  ([#16]).
+- Prologue: the Big-O lab's 2ⁿ values, one reference metaphor, the memory
+  street and the cheat sheet ([#18]).
+- Every chapter's corrections, cross-references and register: array ([#19]),
+  string ([#20]), linked list ([#21]), stack ([#22]), queue ([#23]),
+  hash table ([#24]), binary tree ([#25]), BST ([#26]), trie ([#27]),
+  graph ([#28]) and advanced structures ([#29]).
+- Chinese copy no longer shows the stray spaces JSX made from line breaks;
+  a unit test guards the rule ([#39]).
+
+### Interaction and accessibility
+
+- The quiz accepts full-width punctuation and IME input, keeps focus and shows
+  the best score ([#17]).
+- The sidebar drawer, toolbar and command palette are keyboard-safe; the
+  sidebar no longer prefetches every chapter ([#30]).
+- The union-find, graph and segment-tree labs work with the keyboard ([#15],
+  [#28], [#29]).
+- Text and controls meet WCAG AA contrast in both themes ([#32]).
+- Code renders as typed (no ligatures), uses one monospace font, keeps its
+  file name on phones and has real tab semantics ([#33]).
+
+### Layout
+
+- Each page has its own title, and unknown paths get a real 404 page ([#31]).
+- Phone layouts stay inside the screen, the overflow test can fail, and
+  printing shows every section ([#34]).
+- Text fields are 16px on phones, so iOS does not zoom in ([#40]).
+
+### Performance
+
+- Idle pages no longer keep the main thread busy; the home-page morph can be
+  paused and respects reduced motion ([#35]).
+- Fonts are self-hosted, so builds no longer fail when Google Fonts returns an
+  unexpected URL, and English pages no longer load the Chinese font ([#37]).
+- The atlas loads the chapters' problems without their quizzes ([#38]).
+
+### Not changed
+
+- English is the default language and readers switch to Chinese by hand; a
+  reader who chose Chinese briefly sees English on first paint.
+
+[#11]: https://github.com/renrenmimi/DataData/pull/11
+[#12]: https://github.com/renrenmimi/DataData/pull/12
+[#13]: https://github.com/renrenmimi/DataData/pull/13
+[#14]: https://github.com/renrenmimi/DataData/pull/14
+[#15]: https://github.com/renrenmimi/DataData/pull/15
+[#16]: https://github.com/renrenmimi/DataData/pull/16
+[#17]: https://github.com/renrenmimi/DataData/pull/17
+[#18]: https://github.com/renrenmimi/DataData/pull/18
+[#19]: https://github.com/renrenmimi/DataData/pull/19
+[#20]: https://github.com/renrenmimi/DataData/pull/20
+[#21]: https://github.com/renrenmimi/DataData/pull/21
+[#22]: https://github.com/renrenmimi/DataData/pull/22
+[#23]: https://github.com/renrenmimi/DataData/pull/23
+[#24]: https://github.com/renrenmimi/DataData/pull/24
+[#25]: https://github.com/renrenmimi/DataData/pull/25
+[#26]: https://github.com/renrenmimi/DataData/pull/26
+[#27]: https://github.com/renrenmimi/DataData/pull/27
+[#28]: https://github.com/renrenmimi/DataData/pull/28
+[#29]: https://github.com/renrenmimi/DataData/pull/29
+[#30]: https://github.com/renrenmimi/DataData/pull/30
+[#31]: https://github.com/renrenmimi/DataData/pull/31
+[#32]: https://github.com/renrenmimi/DataData/pull/32
+[#33]: https://github.com/renrenmimi/DataData/pull/33
+[#34]: https://github.com/renrenmimi/DataData/pull/34
+[#35]: https://github.com/renrenmimi/DataData/pull/35
+[#36]: https://github.com/renrenmimi/DataData/pull/36
+[#37]: https://github.com/renrenmimi/DataData/pull/37
+[#38]: https://github.com/renrenmimi/DataData/pull/38
+[#39]: https://github.com/renrenmimi/DataData/pull/39
+[#40]: https://github.com/renrenmimi/DataData/pull/40

README.md

@@ -67,12 +67,15 @@ Next.js 15 (App Router) + TypeScript + React 19, plain CSS. There are no API rou
 pages.
 
 Each chapter is one folder under `app/` holding its page, its visualizations (`viz.tsx`) and
-its own stylesheet, paired with a data file under `lib/` for the problem sets.
+its own stylesheet, paired with two data files under `lib/`: its problem set
+(`<chapter>-problems.tsx`) and its quiz (`<chapter>-quiz.tsx`).
 
 Shared pieces: the frame-by-frame player in `lib/stepper.tsx`, the quiz component in
 `lib/quiz.tsx`, progress tracking in `lib/progress.tsx`, and design tokens in
 `app/globals.css`.
 
+Changes are recorded in [`CHANGELOG.md`](CHANGELOG.md).
+
 ---
 
 © 2026 Weiren Feng. All rights reserved. Published for reading and portfolio purposes; not