Bug fixes:
- Fix data race on client->width/height (now _Atomic int)
- Persist join/leave system messages via message_save()
- Make room_add_message static to enforce lock contract
- Fix execute_command mutating command_input directly
- Increase help_copy buffer from 4096 to 8192 for CJK safety
New features:
- Add :msg/:w whisper command for private messaging
- Add command history with UP/DOWN arrows in command mode
- Add Ctrl+D/U/F/B page scrolling in normal mode
- Add :q/:quit/:exit Vim-style disconnect
Unix community:
- Add tnt.1 manpage (roff format) with full documentation
- Add manpage install/uninstall to Makefile
- Add 11 unit tests for chat_room.c covering: create/destroy, message
add/overflow, broadcast sequence, get_message bounds, client
add/remove/capacity, and null argument handling
- Add unit-test target to root Makefile so `make test` runs unit tests
before integration tests
- Add common.c to unit test link dependencies (needed for tnt_state_path)
- Guard _DARWIN_C_SOURCE define to prevent -Wmacro-redefined warning
