feat: refactor Nakama plugin into generic multi-game match engine

### Highlights
- Introduced generic match engine (`generic_match.go`) implementing dynamic GameRules-based runtime.
- Added modular structure under `/plugins`:
  - /plugins/game      → GameRules interface + TicTacToe + Battleship rule sets
  - /plugins/structs   → Board, Player, MatchState generic structs
  - /plugins/modules   → matchmaking + RPC handlers + match engine
- Migrated TicTacToe logic into reusable rule implementation.
- Added Battleship game support using same engine.
- Updated matchmaking to accept { game, mode } for multi-game routing.
- Updated UI contract: clients must send `game` (and optional `mode`) when joining matchmaking.
- Removed hardcoded TicTacToe match registration.
- Registered a single “generic” authoritative match with ruleset registry.
- Normalized imports under local dev module path.
- Ensured MatchState and Board are now generic and reusable across games.
- Added strict requirement for `game` metadata in match flow (error if missing).
- Cleaned initial state creation into MatchInit with flexible board dimensions.
- Improved MatchLeave for proper forfeit handling through GameRules.

### Result
The server now supports an unlimited number of turn-based board games
via swappable rulesets while keeping a single authoritative Nakama match loop.

plugins/structs/board.go

@@ -0,0 +1,51 @@
+package game
+
+// Board is a generic 2D grid for turn-based games.
+// Cell data is stored as strings, but can represent anything (piece, move, state).
+type Board struct {
+	Rows int      `json:"rows"`
+	Cols int      `json:"cols"`
+	Grid [][]string `json:"grid"`
+}
+
+// NewBoard creates a grid of empty strings.
+func NewBoard(rows, cols int) *Board {
+	b := &Board{
+		Rows: rows,
+		Cols: cols,
+		Grid: make([][]string, rows),
+	}
+
+	for r := 0; r < rows; r++ {
+		b.Grid[r] = make([]string, cols)
+	}
+
+	return b
+}
+
+func (b *Board) InBounds(row, col int) bool {
+	return row >= 0 && row < b.Rows && col >= 0 && col < b.Cols
+}
+
+func (b *Board) Get(row, col int) string {
+	return b.Grid[row][col]
+}
+
+func (b *Board) Set(row, col int, value string) {
+	b.Grid[row][col] = value
+}
+
+func (b *Board) IsEmpty(row, col int) bool {
+	return b.Grid[row][col] == ""
+}
+
+func (b *Board) Full() bool {
+	for r := 0; r < b.Rows; r++ {
+		for c := 0; c < b.Cols; c++ {
+			if b.Grid[r][c] == "" {
+				return false
+			}
+		}
+	}
+	return true
+}

plugins/structs/match_state.go

@@ -0,0 +1,10 @@
+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
+}

plugins/structs/player.go

@@ -0,0 +1,19 @@
+package game
+
+// Player represents a participant in the match.
+type Player struct {
+	UserID   string            `json:"user_id"`
+	Username string            `json:"username"`
+	Index    int               `json:"index"`
+	Metadata map[string]string `json:"metadata,omitempty"`
+}
+
+// NewPlayer creates a new player object.
+func NewPlayer(userID, username string, index int) *Player {
+	return &Player{
+		UserID:   userID,
+		Username: username,
+		Index:    index,
+		Metadata: make(map[string]string),
+	}
+}