Moonchess: vanishing tic-tac-toe, online and practice modes

Online play over WebSockets with any-string game codes, seats and spectators,
and a 1-second heartbeat. Practice mode against Columbina, backed by a full
solver, with an accuracy slider, spoilers, undo and piece freshness.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

static/index.html

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+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Moonchess</title>
+  <link rel="stylesheet" href="/static/style.css">
+</head>
+<body>
+  <main>
+    <section id="lobby">
+      <h1>Moonchess</h1>
+      <p class="lede">Tic-tac-toe, but you only get three pieces. Place a fourth and your oldest one vanishes first.</p>
+      <button id="create" class="primary">Generate code</button>
+      <div class="divider"><span>or enter any code</span></div>
+      <form id="join-form">
+        <input id="join-code" placeholder="Any code or word" maxlength="32" autocomplete="off"
+               autocapitalize="characters" spellcheck="false" aria-label="Game code">
+        <button type="submit">Join</button>
+      </form>
+      <p id="lobby-error" class="error" role="alert"></p>
+    </section>
+
+    <section id="game" hidden>
+      <header class="bar">
+        <div class="code-wrap">
+          <span class="label">Code</span>
+          <span id="code" class="code"></span>
+          <button id="copy" class="small">Copy link</button>
+        </div>
+        <button id="leave" class="small ghost">Leave</button>
+      </header>
+
+      <div class="players">
+        <div class="player" id="player-0"><span class="mark x">X</span><span class="who"></span></div>
+        <div class="player" id="player-1"><span class="mark o">O</span><span class="who"></span></div>
+      </div>
+
+      <p id="status" class="status" aria-live="polite"></p>
+
+      <div id="board" class="board" role="grid" aria-label="Board"></div>
+
+      <div class="footer">
+        <button id="rematch" class="primary" hidden>Rematch</button>
+        <p id="meta" class="meta"></p>
+      </div>
+    </section>
+  </main>
+  <script src="/static/board.js"></script>
+  <script src="/static/app.js"></script>
+</body>
+</html>