feat: 3D terminal emulator in Batcave alcove (#269)

Adds a WebGL Batcave terminal — a cave-like alcove with a console desk,
monitor frame, and canvas-texture terminal screen rendered in Three.js.

- Press [B] to fly camera to Batcave and focus the terminal
- Full keyboard capture with cursor blink and scanline CRT aesthetic
- Built-in commands: help, clear, status, agents, whoami, uptime, date,
  ls, ping, exit/quit
- Terminal input is isolated from scene key-bindings (Tab/P/sovereignty)
  while focused
- Green phosphor glow light pulses from the screen into the cave

Fixes #269

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -48,6 +48,11 @@
 
   <div id="sovereignty-msg">⚡ SOVEREIGNTY ⚡</div>
 
+  <div id="batcave-indicator">
+    <span>BATCAVE TERMINAL</span>
+    <span class="batcave-hint">[B] exit &nbsp;|&nbsp; [Esc] exit &nbsp;|&nbsp; type commands below</span>
+  </div>
+
   <script>
     if ('serviceWorker' in navigator) {
       navigator.serviceWorker.register('/sw.js').catch(() => {});