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/structs/match_state.go

@@ -2,9 +2,9 @@ package structs
 
 // MatchState holds the full game session state.
 type MatchState struct {
-    Players  []*Player `json:"players"`
-    Board    *Board    `json:"board"`
-    Turn     int       `json:"turn"`       // index in Players[]
-    Winner   int       `json:"winner"`     // -1 = none, >=0 = winner index
-    GameOver bool      `json:"game_over"`  // true when the match ends
+  Players  []*Player           `json:"players"`
+  Boards map[string]*Board     `json:"boards"`      // Multiple named boards:
+  Turn     int                 `json:"turn"`        // index in Players[]
+  Winner   int                 `json:"winner"`      // -1 = none, >=0 = winner index
+  GameOver bool                `json:"game_over"`   // true when the match ends
 }