feat(battleship,tictactoe,engine): add multi-board support and keepTurn logic

### Core Engine
- Updated `GameRules.ApplyMove` to return `(changed, gameOver, winnerIdx, keepTurn)`
- Added keepTurn handling in `MatchLoop` to support Battleship mode B (classic rules)
- Removed old single-board handling from MatchState and MatchInit
- Cleaned go.mod by marking protobuf dependency as indirect

### Battleship
- Implemented board-based state tracking using MatchState.Boards:
  - `p0_ships`, `p0_shots`, `p1_ships`, `p1_shots`
- Removed legacy metadata-based ship/shot board encoding
- Rewrote ValidateMove to use structured boards
- Rewrote ApplyMove for classic Battleship rules (mode B):
  - Hits allow the attacker to keep their turn
  - Miss switches turn
  - Destroyed ship sections marked `X`
- Improved CheckGameOver using structured boards

### TicTacToe
- Updated ApplyMove signature to match new interface
- Ensured TicTacToe always returns `keepTurn = false`
- Updated code paths to use MatchState.Boards instead of Board

### Summary
This commit completes the migration from a single-board architecture to a
multi-board architecture across the engine, TicTacToe, and Battleship, enabling
support for more complex games and multiple modes such as Battleship Mode B.

plugins/games/rules.go

@@ -22,7 +22,11 @@ type GameRules interface {
 
     // Apply a move.
     // Returns: (changed, gameOver, winnerIndex)
-    ApplyMove(state *structs.MatchState, playerIdx int, payload MovePayload) (bool, bool, int)
+    ApplyMove(
+        state *structs.MatchState,
+        playerIdx int,
+        payload MovePayload,
+    ) (changed bool, gameOver bool, winnerIdx int, keepTurn bool)
 
     // If a player leaves, who wins?
     // Return: