feat(disk): raise density_freq default to 2.0

Higher default turbulence frequency gives the volumetric disk more
trackable texture features at startup, making the Keplerian rotation
(rot ∝ 1/r^1.5) visually readable without needing to open the egui
panel. The old 0.8 default produced a near-featureless smooth slab at
default settings — the disk was rotating but the eye had nothing to lock
onto. 2.0 sits in the upper half of the slider range (0.2–3.0) while
still leaving room to push higher for debugging.

src/params.rs

@@ -189,7 +189,7 @@ impl Default for BlackHoleParams {
             disk_half_thickness: 0.15,
             filament_freq: 1.0,
             filament_sharpness: 2.0,
-            density_freq: 0.8,
+            density_freq: 2.0,
             // Raised from 1.0: the new density model has no 0.55 floor and decays
             // to 0 at the edges, so a slightly higher multiplier keeps the bulk
             // opaque after per-step integration.

src/render/material.rs

@@ -101,7 +101,7 @@ impl Default for BlackHoleUniforms {
             disk_half_thickness: 0.15,
             filament_freq: 1.0,
             filament_sharpness: 2.0,
-            density_freq: 0.8,
+            density_freq: 2.0,
             density_strength: 1.2,
             arm_count: 2.0,
             arm_tightness: 2.0,