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/tic_tac_toe.go

@@ -70,10 +70,10 @@ func (t *TicTacToeRules) ApplyMove(
   state *structs.MatchState,
   playerIdx int,
   payload MovePayload,
-) (bool, bool, int) {
+) (bool, bool, int, bool) {
   b := state.Boards["tictactoe"]
   if b == nil {
-      return false, false, -1
+      return false, false, -1, false
   }
 
   symbol := state.Players[playerIdx].Metadata["symbol"]
@@ -85,7 +85,7 @@ func (t *TicTacToeRules) ApplyMove(
 
   over, winner := t.CheckGameOver(state)
 
-  return true, over, winner
+  return true, over, winner, false
 }