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/games/tic_tac_toe.go

@@ -0,0 +1,143 @@
+package game
+
+import (
+    "errors"
+    "fmt"
+    "localrepo/plugins/structs"
+)
+
+// TicTacToeRules implements GameRules for 3x3 Tic Tac Toe.
+type TicTacToeRules struct{}
+
+// -------------------------------
+// GameRules Implementation
+// -------------------------------
+
+func (t *TicTacToeRules) MaxPlayers() int {
+    return 2
+}
+
+// Assign player symbols: X and O
+func (t *TicTacToeRules) AssignPlayerSymbols(players []*Player) {
+    if len(players) < 2 {
+        return
+    }
+
+    players[0].Metadata["symbol"] = "X"
+    players[1].Metadata["symbol"] = "O"
+}
+
+// ValidateMove checks bounds and empty cell.
+func (t *TicTacToeRules) ValidateMove(state *structs.MatchState, playerIdx int, payload MovePayload) bool {
+
+    rowVal, ok1 := payload.Data["row"]
+    colVal, ok2 := payload.Data["col"]
+
+    if !ok1 || !ok2 {
+        return false
+    }
+
+    row, ok3 := rowVal.(float64)
+    col, ok4 := colVal.(float64)
+
+    if !ok3 || !ok4 {
+        return false
+    }
+
+    r := int(row)
+    c := int(col)
+
+    // bounds
+    if !state.Board.InBounds(r, c) {
+        return false
+    }
+
+    // empty?
+    return state.Board.IsEmpty(r, c)
+}
+
+// ApplyMove writes X or O to the board.
+func (t *TicTacToeRules) ApplyMove(state *structs.MatchState, playerIdx int, payload MovePayload) {
+
+    symbol := state.Players[playerIdx].Metadata["symbol"]
+
+    r := int(payload.Data["row"].(float64))
+    c := int(payload.Data["col"].(float64))
+
+    state.Board.Set(r, c, symbol)
+}
+
+// CheckGameOver determines win/draw state.
+func (t *TicTacToeRules) CheckGameOver(state *structs.MatchState) (bool, int) {
+
+    winnerSymbol := t.findWinner(state.Board)
+
+    if winnerSymbol != "" {
+        // find the player with this symbol
+        for _, p := range state.Players {
+            if p.Metadata["symbol"] == winnerSymbol {
+                return true, p.Index
+            }
+        }
+        return true, -1
+    }
+
+    if state.Board.Full() {
+        return true, -1 // draw
+    }
+
+    return false, -1
+}
+
+// OnForfeit: whoever leaves loses instantly
+func (t *TicTacToeRules) ForfeitWinner(state *structs.MatchState, leaverIndex int) int {
+
+    // If player 0 leaves, player 1 wins.
+    if leaverIndex == 0 && len(state.Players) > 1 {
+        return 1
+    }
+
+    // If player 1 leaves, player 0 wins.
+    if leaverIndex == 1 && len(state.Players) > 0 {
+        return 0
+    }
+
+    // Otherwise draw.
+    return -1
+}
+
+// -------------------------------
+// Helper: winner detection
+// -------------------------------
+
+func (t *TicTacToeRules) findWinner(b *structs.Board) string {
+
+    lines := [][][2]int{
+        // rows
+        {{0, 0}, {0, 1}, {0, 2}},
+        {{1, 0}, {1, 1}, {1, 2}},
+        {{2, 0}, {2, 1}, {2, 2}},
+        // cols
+        {{0, 0}, {1, 0}, {2, 0}},
+        {{0, 1}, {1, 1}, {2, 1}},
+        {{0, 2}, {1, 2}, {2, 2}},
+        // diagonals
+        {{0, 0}, {1, 1}, {2, 2}},
+        {{0, 2}, {1, 1}, {2, 0}},
+    }
+
+    for _, line := range lines {
+        r1, c1 := line[0][0], line[0][1]
+        r2, c2 := line[1][0], line[1][1]
+        r3, c3 := line[2][0], line[2][1]
+
+        v1 := b.Get(r1, c1)
+        if v1 != "" &&
+            v1 == b.Get(r2, c2) &&
+            v1 == b.Get(r3, c3) {
+            return v1
+        }
+    }
+
+    return ""
+}