docs(plan): volumetric accretion disk implementation plan
9-task plan for the volumetric disk spec. Tasks are ordered for
independent compilability (a temporary disk_color shim in Task 5 keeps
the build green until Task 7 restructures the main loop):
1. DiskQuality enum + 8 params + defaults (params.rs)
2. 9 uniform fields, Rust + WGSL in lockstep (material.rs, shader)
3. mirror_params copies (plugin.rs)
4. ridged_fbm noise (shader, inert)
5. shared helpers + DiskSample + disk_color_flat (shader refactor)
6. disk_color_volumetric: ridged + density + spiral arms (shader, inert)
7. main-loop restructure: in-slab sampling + edge-capture + tier dispatch
8. Disk Turbulence egui panel (ui.rs)
9. full verification: cargo test, desktop + web visual + perf check
xfy
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