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/battleship.go

@@ -62,6 +62,21 @@ type BattleshipRules struct{}
 
 func (b *BattleshipRules) MaxPlayers() int { return 2 }
 
+func (b *BattleshipRules) InitBoards(players []*structs.Player, cfg GameConfiguration) map[string]*structs.Board {
+    boards := make(map[string]*structs.Board)
+    // One ships board and one shots board per player
+    for _, p := range players {
+        pid := fmt.Sprintf("p%d", p.Index)
+
+        // Player's fleet board (ships placement)
+        boards[pid+"_ships"] = structs.NewBoard(cfg.Board.Rows, cfg.Board.Cols)
+
+        // Player's attack tracking board (shots fired at opponent)
+        boards[pid+"_shots"] = structs.NewBoard(cfg.Board.Rows, cfg.Board.Cols)
+    }
+    return boards
+}
+
 // ------------------------------
 // Assign player boards
 // ------------------------------