Design for replacing the smooth zero-thickness disk with a Gargantua-style
volumetric gas disk. Supersedes the Phase 3 non-goal "disk stays a zero-
thickness plane" - user feedback identified zero thickness as a root cause
of the disk looking too smooth.
Key decisions captured:
- Approach A volumetric integration: in-slab sampling reuses the existing
RK45 adaptive step (dense where light bends, sparse where straight),
no separate ray-march. Midplane edge-capture retained for straddling steps.
- Ridged multifractal noise (ridge^2) for sharp bright filaments, replacing
the smooth FBM blobs that produce the current mushy texture.
- Separate density field (smoothstep-gated FBM) for gas clumping, plus a
logarithmic-spiral arm modulation riding the Keplerian shear.
- DiskQuality tier enum (Off/Low/Medium/High) gates octave counts; Off is
a full revert to the flat disk as perf escape hatch + visual A/B.
- physics.rs untouched: volumetric integration is render-sampling layer,
never enters the geodesic solver; existing tests stay green.
Design for raising the renderer from geometrically-correct-but-flat to
Gargantua-reference fidelity. Four user-selected traits:
- HDR color + ACES tone mapping (Rgba16Float offscreen, Narkowicz fit)
- Multi-pass HDR bloom (bright-pass → down-pyramid → up-pyramid → composite)
- Domain-warped FBM disk texture (replaces two-sine noise)
- Anti-aliasing via render_scale + gaussian-speck stars
All quality options configurable via a dedicated egui Quality panel,
with tiered web defaults (Low bloom, no MSAA/star-AA) vs desktop (High).
BloomQuality::Off falls back to the original LDR path for WebGPU
browsers without float-filterable support.
Geodesic integrator (deriv/rk45_step/is_captured_rk45) untouched —
the physics.rs mirror contract is unaffected. No new tests.
- Material2d bind group is group 2 (#{MATERIAL_BIND_GROUP}), not group 1
- RenderPlugin has no wgpu_backends field; enable bevy 'webgpu' feature instead
- WgpuSettings::default() auto-selects BROWSER_WEBGPU on wasm32
- Imports from bevy::sprite_render (Material2d, AlphaMode2d)
- Component-based spawning (Camera2d, Mesh2d/MeshMaterial2d), no bundles
- getrandom 0.4/wasm_js handled transitively by Bevy
