Add a 휴게실 tank game tab under the literature list

The crossword shell only carried literature tabs. Add a second nav
section, 휴게실, holding a standalone canvas tank game: two stages
(fortress with 4 enemies, maze with 6), arrow keys to move, space to
fire, WebAudio for the shots and explosions.

The page shares nothing with the crossword engine, so the Dockerfile's
asset-hash stamp leaves it alone. Inside the shell's iframe a keydown
only lands once the frame has focus, so the canvas takes a tabindex and
the click handler focuses it; the start overlay says to click first.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

site/index.html

@@ -51,6 +51,9 @@ body{
   font-size:10px;font-weight:900;letter-spacing:.28em;color:var(--slate);
   padding:0 20px 10px;
 }
+.nav .label.rest{
+  margin-top:14px;padding-top:15px;border-top:1px solid var(--line);
+}
 .nav a{
   display:block;text-decoration:none;color:inherit;
   padding:12px 18px 12px 17px;border-left:3px solid transparent;
@@ -147,6 +150,12 @@ body{
       <span class="kr">일본<span class="suffix"> 문학</span></span>
       <span class="og">日本文学</span>
     </a>
+
+    <div class="label rest">휴게실</div>
+    <a href="tank.html" target="stage" data-key="tank">
+      <span class="kr">탱크게임</span>
+      <span class="og">Tank Battle</span>
+    </a>
   </div>
 
   <div class="rail-foot">
@@ -167,7 +176,8 @@ var PAGES = {
   british: {file:'british.html', name:'영국 문학'},
   french:  {file:'french.html',  name:'프랑스 문학'},
   german:  {file:'german.html',  name:'독일 문학'},
-  japanese:{file:'japanese.html', name:'일본 문학'}
+  japanese:{file:'japanese.html', name:'일본 문학'},
+  tank:    {file:'tank.html',     name:'휴게실 · 탱크게임'}
 };
 var DEFAULT = 'korean';