docs(spec): add desktop + web (WebGPU) platform support
- WebGPU-only web target (Bevy forces WebGPU/WebGL2 at build time; WebGL2
lacks storage buffers our planets feature needs)
- Add §13 Platform support: build toolchain (trunk/wasm32), backend gating,
fallback message, web defaults (render_scale 0.75, steps 200)
- Add src/web.rs, web/index.html, Trunk.toml to file structure
- Add web-sys dep; add web build + perf verification milestones
- Update storage-buffer risk note: valid on native + WebGPU
xfy
c1bfe2b25e