Add Emscripten WebGPU backend in thirteen.h

[fcarucci]

Summary

Adds WebAssembly support to Thirteen using a WebGPU backend implemented in thirteen.h, and updates the Emscripten build flow in readme.md.

Changes

• Added __EMSCRIPTEN__ support path in thirteen.h.
• Implemented PlatformWeb for browser input/canvas handling:
  • keyboard + mouse mapped to existing Thirteen input state.
  • canvas resize integration with Thirteen sizing.
• Implemented RendererWebGPU in thirteen.h:
  • creates surface from #canvas.
  • async adapter/device setup with wgpuInstanceRequestAdapter and wgpuAdapterRequestDevice.
  • uploads Thirteen RGBA pixel buffer each frame to the WebGPU surface texture.
  • handles resize reconfiguration.
• Added Emscripten frame yield in Render() (emscripten_sleep(0) under __EMSCRIPTEN__) so browser events/promises progress with native-style sample loops.
• Updated backend selection so Emscripten uses the new web platform/renderer.
• Updated web build instructions in readme.md to current flags (--use-port=emdawnwebgpu, -sASYNCIFY).

Build

emcc Examples/Simple/main.cpp -std=c++17 -I. \
  --use-port=emdawnwebgpu \
  -sASYNCIFY \
  -sALLOW_MEMORY_GROWTH=1 \
  -o simple.html

[Atrix256]

Looking good! i might re-arrange things in the readme a bit after merging.