Restructures the disk block in the main loop: - Off tier: flat disk_color_flat single midplane sample (old behavior). - Volumetric tiers: (A) in-slab per-step sampling accumulates emission × arc length, reusing the RK45 adaptive step density; (B) midplane edge- capture weights one at-plane sample by slab depth along the ray. Removes the Task-5 disk_color shim. physics.rs untouched; cargo test green.