27 tests covering: welcome/setNickname/home transitions, runner exit on
ErrClientExit, waiting owner/joiner role split with StartCh signal,
PVP move validation (turn order, bounds, occupancy, win detection with
GameOverCh cross-goroutine sync), PVE human+AI alternation and AI-first
when human is White, gameover reset/rematch for both PVP and PVE.
Remove database/legacy.go (legacy hashmap store, Room/Gomoku types).
Remove state/create.go and state/join.go (collapsed into home.go).
Remove state/game/gomoku.go (replaced by game_pvp.go + game_shared.go).
Remove server/pkg/{model,network,protocol,async,json,strings} packages
that are no longer imported after the state machine rewrite.
Stub out network/network.go (legacy TCP handler retired).
Rewrite database/player.go to remove legacy conn/data fields.