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/structs/match_state.go

@@ -7,5 +7,5 @@ type MatchState struct {
   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
-  Metadata map[string]interface{} `json:metadata`     // metadata
+  Metadata map[string]interface{} `json:"metadata"`      // metadata
 }