Add diegetic NPC dialogue demo — interactive HTML in a Three.js scene (#71)

[Twynzen]

Summary

Adds an interactive HTML dialogue panel rendered as a texture on a 3D billboard plane in a Three.js scene, using the new texElementImage2D API. Addresses #71 by providing an interactive
alternative to the existing webGL cube demo.

What it showcases

• Real DOM controls — <select>, <input>, and <button> with full keyboard and
  screen-reader access, painted into the 3D scene.
• Stable hit-testing under orbit — OrbitControls camera lets the user rotate around the NPC
  while all controls remain responsive, including <input> focus and typing.
• Three.js integration pattern — the demo documents how to work around the friction between
  texElementImage2D (spec'd like texImage2D) and Three.js' default texStorage2D immutable
  storage, by creating the GL handle manually and injecting it into the library's texture cache.

How to test

1. Open in Chrome Canary with chrome://flags/#canvas-draw-element enabled.
2. Serve the repo locally and open Examples/diegetic-npc-dialogue.html.
3. Try the select dropdown, type in the input, and orbit the camera with mouse drag — interaction
   should remain stable in any viewing angle.
4. Keyboard: Tab cycles through select → input → button. Arrow keys change the select value. Typing updates the rendered texture.

Notes on integration friction (feedback for #94)

While building the demo I hit three issues worth documenting — I can open a separate comment on #94 with the full analysis, but briefly:

1. Three.js allocates textures as immutable (texStorage2D), which fails texElementImage2D.
   Workaround: bypass the library's allocation via renderer.properties.get(tex).__webglTexture.
2. Naively computing the CSS transform from getBoundingClientRect() is self-referential and
   drifts. Measuring a natural-position anchor once and computing absolute transforms fixes it.
3. Library camera controls (OrbitControls, etc.) attached to the canvas capture pointerdown from
   interactive DOM descendants — stopPropagation() on the panel is required.

Happy to expand on any of these if helpful.