feat(rules): add game-specific metadata attachment and unify match metadata initialization

Added AttachGameMetadata to GameRules interface

Implemented metadata setup for Battleship (phase + readiness flags)

Implemented no-op metadata hook for TicTacToe

Moved generic phase/ready metadata out of MatchInit

Added game/mode metadata to match state

Fixed json:"metadata" tag in MatchState

plugins/games/tictactoe.go

@@ -31,6 +31,13 @@ func (t *TicTacToeRules) AssignPlayerSymbols(players []*structs.Player) {
     players[1].Metadata["symbol"] = "O"
 }
 
+// ------------------------------
+// Attach Game Metadata
+// ------------------------------
+func (b *TicTacToeRules) AttachGameMetadata(state *structs.MatchState) {
+    // nothing needed for tictactoe
+}
+
 // ValidateMove checks bounds and empty cell.
 func (t *TicTacToeRules) ValidateMove(state *structs.MatchState, playerIdx int, payload MovePayload) bool {