fix: multiplayer table routing, backgammon state merge

Phase 2.4: multiplayer.php now routes to correct table for all games
(chess→matches, domino→domino_matches, backgammon→backgammon_matches,
ludo→ludo_matches). Validates game_key against allowed list.

Phase 4.3: handleServerState in backgammon only copies specific fields
(state, dice, turn, gameOver) instead of Object.assign which clobbered
local methods and properties.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api/multiplayer.php

@@ -14,10 +14,22 @@ $input = getInput();
 $action = $input['action'] ?? '';
 $gameKey = $input['game_key'] ?? '';
 
+$validGames = ['chess', 'ludo', 'domino', 'backgammon'];
+if ($gameKey && !in_array($gameKey, $validGames)) { jsonError('Invalid game_key', 400); }
+
+function resolveTable(string $gameKey): string {
+    return match($gameKey) {
+        'chess' => 'matches',
+        'domino' => 'domino_matches',
+        'backgammon' => 'backgammon_matches',
+        default => 'ludo_matches',
+    };
+}
+
 switch ($action) {
     case 'create_room':
         $db = supabase($token);
-        $table = $gameKey === 'domino' ? 'domino_matches' : 'ludo_matches';
+        $table = resolveTable($gameKey);
         $result = null;
         $code = '';
         for ($attempt = 0; $attempt < 5; $attempt++) {
@@ -40,7 +52,7 @@ switch ($action) {
         $roomCode = $input['room_code'] ?? '';
         if (!$roomCode) jsonError('room_code required');
         $sdb = supabaseService();
-        $table = $gameKey === 'domino' ? 'domino_matches' : 'ludo_matches';
+        $table = resolveTable($gameKey);
         $match = $sdb->get($table, ['room_code' => 'eq.' . $roomCode, 'status' => 'eq.waiting', 'select' => 'id,players,player_count', 'limit' => 1]);
         if (!is_array($match) || empty($match) || isset($match['error'])) jsonError('Room not found', 404);
         $match = $match[0];

public/js/modules/backgammon/scenes/game.js

@@ -633,7 +633,11 @@ function handleServerState(data) {
   if (!data || data.error) return;
   matchSession.markOpponentActive();
   if (data.game_state) {
-    Object.assign(game, data.game_state);
+    const gs = data.game_state;
+    if (gs.state) game.state = gs.state;
+    if (gs.dice) game.dice = gs.dice;
+    if (gs.turn !== undefined) game.turn = gs.turn;
+    if (gs.gameOver !== undefined) game.gameOver = gs.gameOver;
     if (data.match_state) match.scores = data.match_state.scores || match.scores;
     if (data.cube) match.cube = data.cube;
     updateUI();