Several worlds: a Worlds list with load, rename and delete; old save migrates to World 1

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/shell.html

@@ -47,6 +47,7 @@
     <p class="tagline">Dig. Smelt. Cast your own tools. Every grain is simulated, and nothing turns into anything unless the physics says so.</p>
     <div class="tmenu" id="tmenu">
       <button class="btn primary" id="btn-continue" hidden>Continue <small id="continue-meta"></small></button>
+      <button class="btn" id="btn-worlds" hidden>Worlds <small>load · rename · delete</small></button>
       <button class="btn" id="btn-new">New world <small>generate</small></button>
       <div class="plate newworld" id="newworld" hidden>
         <label>Seed <input id="nw-seed" type="text" inputmode="numeric" autocomplete="off"></label>
@@ -101,6 +102,13 @@
   </div>
 </div>
 
+<div id="m-worlds" class="modal" hidden>
+  <div class="plate">
+    <div class="mhead"><h2>Worlds</h2><button class="iconbtn" data-close aria-label="Close worlds">Close <kbd>Esc</kbd></button></div>
+    <div class="mbody" id="worlds-list"></div>
+  </div>
+</div>
+
 <div id="m-dialog" class="modal small" hidden>
   <div class="plate">
     <div class="mhead"><h2 id="dlg-title"></h2></div>