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.
