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/lib.rs

@@ -245,6 +245,8 @@ impl Plugin for Codename206 {
                 }
                 band_set[b].makeup_db = band_params[b].makeup_db.smoothed.next();
                 band_set[b].mix = band_params[b].mix.smoothed.next();
+                band_set[b].low_slope = band_params[b].low_slope.smoothed.next();
+                band_set[b].low_curve = band_params[b].low_curve.smoothed.next();
                 self.comps[b].process(&band_in[b][..n], &mut band_out[b][..n], &band_set[b]);
                 for ch in 0..n {
                     summed[ch] += band_out[b][ch];
@@ -273,6 +275,8 @@ impl Plugin for Codename206 {
             }
             all_set.makeup_db = self.params.all.makeup_db.smoothed.next();
             all_set.mix = self.params.all.mix.smoothed.next();
+            all_set.low_slope = self.params.all.low_slope.smoothed.next();
+            all_set.low_curve = self.params.all.low_curve.smoothed.next();
             self.comps[ALL].process(&summed[..n], &mut out_frame[..n], &all_set);
 
             // Output brickwall limiter.