- Add go-nanoid to generate 6-char per-connection IDs; fallback to RemoteAddr
- Introduce sanitizeUsername with rules: [A-Za-z0-9_.-], spaces→_, trim ._-, max 12 chars
- Support /name <username> to change username with validation and broadcast [rename]
- Greet client with default username user_<id> and usage hint for /name
- Broadcast joins/leaves and messages including username and ID; exclude self on join
- Handle /quit to trigger unified leave flow; consistent leave announcement
- Comment/document broadcast struct, Hub, and startTCPServer
- Increase scanner buffer and add newline-delimited send in broadcaster
