feat: serial low-level shaper (Low Slope + Low Curve) before the compressor

Add a per-channel below-threshold shaper composed in series ahead of the comp:
gain = low_shape(level) + comp(level + low_shape(level)). The compressor's
threshold now sees the shaped level, so a Low Slope boost lifts quiet material
up into compression (and a cut pulls it out). Anchored at the -60 dB silence
floor. Low Curve bends the shaper toward a bounded saturation so the serial
composition doesn't blow up (0 = straight line).

gain_computer split into comp_gain_db + low_gain_db and composed; shared with
the editor gain-curve display. Slider order rearranged to read in signal order
(pre-gain -> low shaper -> compressor -> output). Defaults (slope 1, curve 0)
reproduce the plain compressor; 17 tests pass.

Known: the bipolar behaviour isn't final yet (milestone commit).

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

src/params.rs

@@ -68,6 +68,12 @@ pub struct CompressorParams {
     pub ratio: FloatParam,
     #[id = "knee"]
     pub knee_db: FloatParam,
+    /// 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).
+    #[id = "lowcurve"]
+    pub low_curve: FloatParam,
     #[id = "attack"]
     pub attack_ms: FloatParam,
     #[id = "release"]
@@ -168,6 +174,19 @@ impl Default for CompressorParams {
                 .with_unit(" dB")
                 .with_value_to_string(formatters::v2s_f32_rounded(1)),
 
+            low_slope: FloatParam::new(
+                "Low Slope",
+                1.0,
+                FloatRange::Skewed { min: 0.5, max: 3.0, factor: FloatRange::skew_factor(-1.0) },
+            )
+            .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 })
+                .with_smoother(SmoothingStyle::Linear(20.0))
+                .with_value_to_string(formatters::v2s_f32_percentage(0))
+                .with_string_to_value(formatters::s2v_f32_percentage()),
+
             attack_ms: FloatParam::new(
                 "Attack",
                 10.0,
@@ -207,6 +226,8 @@ pub fn build_settings(
         threshold_db: p.threshold_db.value(),
         ratio: p.ratio.value(),
         knee_db: p.knee_db.value(),
+        low_slope: p.low_slope.value(),
+        low_curve: p.low_curve.value(),
         attack_coef: Compressor::time_to_coef(p.attack_ms.value(), sample_rate),
         release_coef: Compressor::time_to_coef(p.release_ms.value(), sample_rate),
         makeup_db: 0.0,