Besiege Earth — low-poly planet in one HTML file

Output EXACTLY ONE complete HTML file. No markdown fences. No explanation before or after the HTML.
The file must run in Chrome with no build step.

Tech stack (required):
- Three.js r170+ from http://cdn.jsdelivr.net
- WebGLRenderer only (NOT WebGPU)
- OrbitControls: orbit a planet at origin; min/max distance clamped

Scene: "Besiege Earth"
- Procedural LOW-POLY planet at origin:
  • Build from icosahedron or cube-sphere (subdivided cube inflated to sphere)
  • Displace vertices with layered simplex/noise for continents and mountains
  • Color by height: deep ocean blue, green lowlands, brown peaks, white ice caps
  • Flat shading (MeshFlatMaterial or computed flat normals) for Besiege-like look
- Thin atmosphere: slightly larger transparent sphere, fresnel/rim glow on edges
- Slow cloud shell: second transparent layer or particle ring, subtle drift
- Surface prop: one medieval SIEGE CATAPULT built from box/cylinder primitives
  (wood brown + metal gray), sitting on land near equator, correctly oriented
  to planet surface normal (not floating, not buried)
- Lighting: warm sun directional + cool ambient; soft gradient sky background
- UI overlay: title "Besiege Earth", subtitle with controls hint
- Animation loop: planet rotates slowly (~0.05 rad/s); optional catapult arm sway
- Responsive full-viewport canvas
- Graceful error message if WebGL unavailable

Constraints:
- All geometry procedural; no external images or models
- Keep total file under 900 lines if possible