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/modules/matchmaking.go

@@ -0,0 +1,101 @@
+package modules
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+
+	"github.com/heroiclabs/nakama-common/runtime"
+)
+
+type MatchmakingTicket struct {
+	Game string `json:"game"` // e.g. "tictactoe", "battleship"
+	Mode string `json:"mode"` // e.g. "classic", "ranked", "blitz"
+}
+
+// --------------------------------------------------
+// GENERIC MATCHMAKER — Supports ALL Games & Modes
+// --------------------------------------------------
+
+func MatchmakerMatched(
+	ctx context.Context,
+	logger runtime.Logger,
+	db *sql.DB,
+	nk runtime.NakamaModule,
+	entries []runtime.MatchmakerEntry,
+) (string, error) {
+
+	if len(entries) == 0 {
+		return "", nil
+	}
+
+	// Extract the first player's desired properties
+	props0 := entries[0].GetProperties()
+
+	game, okGame := props0["game"].(string)
+	mode, okMode := props0["mode"].(string)
+
+	if !okGame || !okMode {
+		logger.Warn("MatchmakerMatched: Missing 'game' or 'mode' properties.")
+		return "", nil
+	}
+
+	// Ensure ALL players match game + mode
+	for _, e := range entries {
+		p := e.GetProperties()
+
+		g, okG := p["game"].(string)
+		m, okM := p["mode"].(string)
+
+		if !okG || !okM || g != game || m != mode {
+			logger.Warn("MatchmakerMatched: Player properties do not match — retrying matchmaking.")
+			return "", nil
+		}
+	}
+
+	// Create the correct authoritative match handler.
+	// This depends on how "game" was registered in main.go.
+	// Example: initializer.RegisterMatch("tictactoe", NewGenericMatch(TicTacToeRules))
+	matchParams := map[string]interface{}{
+		"mode": mode,
+	}
+
+	matchID, err := nk.MatchCreate(ctx, game, matchParams)
+	if err != nil {
+		logger.Error("MatchmakerMatched: MatchCreate failed: %v", err)
+		return "", runtime.NewError("failed to create match", 13)
+	}
+
+	logger.Info("✔ Match created game=%s mode=%s id=%s", game, mode, matchID)
+	return matchID, nil
+}
+
+// --------------------------------------------------
+// RPC: Leave matchmaking (generic cancel API)
+// --------------------------------------------------
+
+func RpcLeaveMatchmaking(
+	ctx context.Context,
+	logger runtime.Logger,
+	db *sql.DB,
+	nk runtime.NakamaModule,
+	payload string,
+) (string, error) {
+
+	var input struct {
+		Ticket string `json:"ticket"`
+	}
+
+	if err := json.Unmarshal([]byte(payload), &input); err != nil {
+		return "", runtime.NewError("invalid JSON", 3)
+	}
+
+	if input.Ticket == "" {
+		return "", runtime.NewError("missing ticket", 3)
+	}
+
+	// Client removes ticket locally — server doesn't need to do anything
+	logger.Info("✔ Player left matchmaking: ticket=%s", input.Ticket)
+
+	return "{}", nil
+}