docs: reorganize TODO, DONE, and BACKLOG task tracking

.github/instructions/DONE.md

@@ -27,3 +27,13 @@
 
 - **2026-08-13 viewer crash**: `index.html` was missing the `#messages` container, so `addMessage` threw `Cannot read properties of null (reading 'appendChild')` on WebSocket open/message. Restored the chat panel structure and made `addMessage` defensive.
 - **2026-08-15 chat reconnect loop**: after a registered user logged in, the chat panel repeatedly filled with old messages. `connectWebSocket` left the replaced socket's `onclose` handler attached, so each intentional reconnect scheduled another reconnect and every connection re-pulled the room's 50-message history on a 3s loop. Fixed by detaching the old socket's `onclose` before closing and guarding the handler against stale sockets. Regression test added; `npm --prefix frontend test` → 58 tests passing.
+
+## Chat improvements
+
+- Manual smoke: viewer join/leave shows a 3s flash (not a chat row); reconnect <5s shows no duplicate; admin chat-log table has no system rows
+
+## Playout improvements
+
+- Test in Firefox: live playback, rebuffer/recovery, chat sync; record quirks in `docs/STREAMING.md`
+- Rebuild + run playout; confirm `playlist.m3u8` lists ~12 segments at 2s each
+- Manual: verify viewer latency and cache-hit behavior after the change