refine(synth): Improve arpeggio velocity control with exponential scaling

Changes the `Arpeggio Velocity Scale` from a linear to an exponential curve (`scale ** 2.5`).

This provides much finer control at lower volume levels, making it easier for users to mix the arpeggiated layer subtly into the background. The new scaling better reflects the human perception of loudness.

app.py

@@ -517,7 +517,14 @@ def arpeggiate_midi(midi_data: pretty_midi.PrettyMIDI, params: AppParameters):
             chord_start_time = min(n.start for n in chord_notes)
             chord_end_time = max(n.end for n in chord_notes)
             avg_velocity = int(np.mean([n.velocity for n in chord_notes]))
-            final_velocity_base = int(avg_velocity * params.s8bit_arpeggio_velocity_scale)
+            
+            # --- Apply an exponential curve to the velocity scale ---
+            # This makes the slider much more sensitive at lower values,
+            # allowing for true background-level arpeggios.
+            scale = params.s8bit_arpeggio_velocity_scale
+            # We use a power of 2 here, but could be tuned (e.g., 1.5, 2.5, 3.0)
+            # A higher power makes the attenuation at low scale values even more aggressive.
+            final_velocity_base = int(avg_velocity * (scale ** 2.5))
             
             if final_velocity_base < 1:
                 final_velocity_base = 1