feat(common-dsp): second-order SVF allpass

SvfCoef::allpass has the phase of a Linkwitz-Riley split at the same
frequency, so a crossover tree can keep its lower bands in phase with
the upper ones. The test checks it against a real LR4 split sample by
sample and for flat magnitude.

plugins/common/dsp/src/svf.rs

@@ -70,6 +70,21 @@ impl SvfCoef {
             m2: -1.,
         }
     }
+    /// Second-order allpass. With `q` = 1/√2 it has the phase of a Linkwitz–Riley crossover
+    /// built from two lowpass and two highpass stages at the same frequency, so it keeps
+    /// the bands of a crossover tree in phase.
+    pub fn allpass(rate: f32, hz: f32, q: f32) -> Self {
+        let k = 1. / q.max(1e-3);
+        let (a1, a2, a3) = Self::base(rate, hz, k, 1.);
+        Self {
+            a1,
+            a2,
+            a3,
+            m0: 1.,
+            m1: -2. * k,
+            m2: 0.,
+        }
+    }
     pub fn high_shelf(rate: f32, hz: f32, q: f32, gain_db: f32) -> Self {
         let a = 10f32.powf(gain_db / 40.);
         let k = 1. / q.max(1e-3);