feat(core): migrate to multi-board architecture and implement per-game InitBoards

### Major changes
- Replace single-board MatchState (`Board`) with multi-board map (`Boards`)
- Update GenericMatch to initialize empty Boards and populate them via GameRules.InitBoards
- Remove legacy `newEmptyBoard` helper from match.go
- Update .gitignore to include *BKP* patterns

### GameRules interface
- Add InitBoards(players, cfg) to allow games to construct their own board sets
- Add detailed documentation explaining method responsibilities and usage
- Improve MovePayload comment clarity

### TicTacToe updates
- Implement InitBoards to produce a single `"tictactoe"` board
- Replace all old references to `state.Board` with `state.Boards["tictactoe"]`
- Make CheckGameOver, ValidateMove, and ApplyMove multi-board compatible

### Battleship updates
- Implement InitBoards generating per-player ships + shots boards:
  - p0_ships, p0_shots
  - p1_ships, p1_shots

### Match flow updates
- Boards are now created only when all players have joined
- Initial state broadcast now includes `boards` instead of `board`

This completes the backend migration for multi-board games and prepares the architecture
for Battleship and other complex board-based games.

plugins/games/tic_tac_toe.go

@@ -15,6 +15,12 @@ func (t *TicTacToeRules) MaxPlayers() int {
     return 2
 }
 
+func (t *TicTacToeRules) InitBoards(players []*structs.Player, cfg GameConfiguration) map[string]*structs.Board {
+    return map[string]*structs.Board{
+        "tictactoe": structs.NewBoard(cfg.Board.Rows, cfg.Board.Cols),
+    }
+}
+
 // Assign player symbols: X and O
 func (t *TicTacToeRules) AssignPlayerSymbols(players []*structs.Player) {
     if len(players) < 2 {
@@ -45,13 +51,18 @@ func (t *TicTacToeRules) ValidateMove(state *structs.MatchState, playerIdx int,
     r := int(row)
     c := int(col)
 
+    b := state.Boards["tictactoe"]
+    if b == nil {
+        return false
+    }
+
     // bounds
-    if !state.Board.InBounds(r, c) {
+    if !b.InBounds(r, c) {
         return false
     }
 
     // empty?
-    return state.Board.IsEmpty(r, c)
+    return b.IsEmpty(r, c)
 }
 
 // ApplyMove writes X or O to the board.
@@ -60,12 +71,17 @@ func (t *TicTacToeRules) ApplyMove(
   playerIdx int,
   payload MovePayload,
 ) (bool, bool, int) {
+  b := state.Boards["tictactoe"]
+  if b == nil {
+      return false, false, -1
+  }
+
   symbol := state.Players[playerIdx].Metadata["symbol"]
 
   r := int(payload.Data["row"].(float64))
   c := int(payload.Data["col"].(float64))
 
-  state.Board.Set(r, c, symbol)
+  b.Set(r, c, symbol)
 
   over, winner := t.CheckGameOver(state)
 
@@ -75,8 +91,12 @@ func (t *TicTacToeRules) ApplyMove(
 
 // CheckGameOver determines win/draw state.
 func (t *TicTacToeRules) CheckGameOver(state *structs.MatchState) (bool, int) {
+    b := state.Boards["tictactoe"]
+    if b == nil {
+        return true, -1 // fallback safety
+    }
 
-    winnerSymbol := t.findWinner(state.Board)
+    winnerSymbol := t.findWinner(b)
 
     if winnerSymbol != "" {
         // find the player with this symbol
@@ -88,7 +108,7 @@ func (t *TicTacToeRules) CheckGameOver(state *structs.MatchState) (bool, int) {
         return true, -1
     }
 
-    if state.Board.Full() {
+    if b.Full() {
         return true, -1 // draw
     }