feat: warp tunnel effect when entering portals (#250)

Adds three floating portal objects (Batcave, Workshop, The Void) around
the platform edge, each with an animated vortex interior shader. Clicking
a portal or pressing W triggers the fullscreen warp tunnel post-processing
effect — a swirling GLSL vortex with ring bands, 8-arm spiral, cyan/purple
color sweep, inward scene distortion, and a white flash at the peak.

- WarpTunnelShader + ShaderPass wired into existing EffectComposer
- Portal inner vortex: custom ShaderMaterial (5-arm spiral + ring bands)
- Raycaster click detection on portal disc meshes
- triggerWarp() state machine: build-up → hold → flash → fade-out
- 'W' key demo cycles through portals
- #warp-indicator HUD element with expanding letter-spacing pulse

Fixes #250

index.html

@@ -47,6 +47,7 @@
   </div>
 
   <div id="sovereignty-msg">⚡ SOVEREIGNTY ⚡</div>
+  <div id="warp-indicator"></div>
 
   <script>
     if ('serviceWorker' in navigator) {