feat(rules): add game-specific metadata attachment and unify match metadata initialization

Added AttachGameMetadata to GameRules interface

Implemented metadata setup for Battleship (phase + readiness flags)

Implemented no-op metadata hook for TicTacToe

Moved generic phase/ready metadata out of MatchInit

Added game/mode metadata to match state

Fixed json:"metadata" tag in MatchState

plugins/games/battleship.go

@@ -89,7 +89,16 @@ func (b *BattleshipRules) InitBoards(players []*structs.Player, cfg GameConfigur
 // Assign player boards
 // ------------------------------
 func (b *BattleshipRules) AssignPlayerSymbols(players []*structs.Player) {
-    // nothing needed for classic mode
+    // nothing needed for battleship
+}
+
+// ------------------------------
+// Attach Game Metadata
+// ------------------------------
+func (b *BattleshipRules) AttachGameMetadata(state *structs.MatchState) {
+  state.Metadata["phase"] = "placement"
+  state.Metadata["p0_ready"] = false
+  state.Metadata["p1_ready"] = false
 }
 
 // ------------------------------