feat: bipolar mid-bulge low-curve anchored at silence and the knee

Rework Low Curve from a one-sided saturating bend into a bipolar mid-bulge: the
low region is anchored at BOTH the silence floor and the knee, Low Slope tilts
the straight line between them, and Low Curve (-1..1) bows that line in the
middle (4*t*(1-t), peak +/-12 dB) without moving either endpoint. Positive
bulges up (boost the quiet middle), negative down (suppress). Still serial.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/params.rs

@@ -71,7 +71,8 @@ pub struct CompressorParams {
     /// Low shaper slope at the silence floor (1 = unity; >1 fans up/boost, <1 fans down/cut).
     #[id = "lowslope"]
     pub low_slope: FloatParam,
-    /// Low shaper curvature (0 = straight line, 1 = max bend toward a bounded saturation).
+    /// Low shaper curvature (−1..1): bipolar mid-bulge, 0 = straight. +bulges up (boost quiet
+    /// middle), − bulges down (suppress). Endpoints (silence + knee) stay fixed.
     #[id = "lowcurve"]
     pub low_curve: FloatParam,
     #[id = "attack"]
@@ -182,7 +183,7 @@ impl Default for CompressorParams {
             .with_smoother(SmoothingStyle::Linear(20.0))
             .with_value_to_string(formatters::v2s_f32_rounded(2)),
 
-            low_curve: FloatParam::new("Low Curve", 0.0, FloatRange::Linear { min: 0.0, max: 1.0 })
+            low_curve: FloatParam::new("Low Curve", 0.0, FloatRange::Linear { min: -1.0, max: 1.0 })
                 .with_smoother(SmoothingStyle::Linear(20.0))
                 .with_value_to_string(formatters::v2s_f32_percentage(0))
                 .with_string_to_value(formatters::s2v_f32_percentage()),