Add private chat to multiplayer rooms

docs/multiplayer.md

@@ -27,12 +27,23 @@ Room state refreshes every two seconds, or every five seconds in a hidden tab.
 Unfinished rooms expire after 24 hours. Multiplayer results are separate from
 the solo leaderboard.
 
+Room chat is available to current players in the lobby, during rounds, and at
+the final standings. Open **Chat** to send a plain-text message of up to 500
+characters. A badge shows unread messages while it is collapsed. Messages
+refresh every two seconds and the latest 100 are restored after a refresh.
+Sending chat does not lock the guess controls or change the round clock.
+Removed players, players who left, and invite visitors cannot read or send
+messages. Closed rooms and rooms older than 24 hours cannot use chat.
+Retries are deduplicated; each player can send at most one new message per
+second and 30 per minute. Player names come from the room identity.
+
 ## Database and verification
 
 Apply `supabase/migrations/20261007071259_multiplayer_rooms.sql` to a new database
 before deploying the web update. It has already been applied to MineGeo's
 configured Supabase project. Deployments continue through `npm run deploy`
 without a Git-provider connection.
+Chat additionally requires `supabase/migrations/20261007155622_multiplayer_chat.sql`.
 
 - `npm test`: validation and existing regression checks.
 - `npm run test:multiplayer`: isolated PostgreSQL state, permission, and scoring