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/rules.go

@@ -2,11 +2,17 @@ package games
 
 import "localrepo/plugins/structs"
 
-// MovePayload is used for incoming move data from clients.
+// MovePayload is the decoded payload sent from clients.
+// It is intentionally untyped (map[string]interface{}) so each game
+// can define its own move structure (e.g., row/col, coordinate, action type, etc.)
 type MovePayload struct {
     Data map[string]interface{} `json:"data"`
 }
 
+// GameRules defines a generic interface for match logic.
+//
+// Each game (TicTacToe, Battleship, Chess, etc.) must implement this interface.
+// The Nakama match handler delegates all game-specific behavior to these methods.
 type GameRules interface {
     // Number of players needed to start.
     MaxPlayers() int
@@ -24,4 +30,17 @@ type GameRules interface {
     //   -1 → draw
     //   -2 → invalid
     ForfeitWinner(state *structs.MatchState, leaverIndex int) int
+
+    // InitBoards initializes all the boards required for the game.
+    //
+    // This is called AFTER all players have joined the match.
+    //
+    // Examples:
+    // - TicTacToe → 1 board shared by both players:   {"tictactoe": 3x3}
+    // - Battleship → 2 boards per player:
+    //       {"p0_ships":10x10, "p0_shots":10x10, "p1_ships":..., "p1_shots":...}
+    //
+    // The returned map is stored in MatchState.Boards.
+    InitBoards(players []*structs.Player, cfg GameConfiguration) map[string]*structs.Board
+
 }