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);

plugins/common/dsp/tests/blocks.rs

@@ -46,6 +46,31 @@ fn svf_bell_at_zero_db_is_an_exact_identity() {
     }
 }
 
+#[test]
+fn svf_allpass_matches_a_linkwitz_riley_split() {
+    let q = std::f32::consts::FRAC_1_SQRT_2;
+    let (low, high, all) = (
+        SvfCoef::lowpass(RATE, 300., q),
+        SvfCoef::highpass(RATE, 300., q),
+        SvfCoef::allpass(RATE, 300., q),
+    );
+    let mut s = [Svf::default(); 5];
+    let mut saw = Saw::new(RATE, 55.);
+    for _ in 0..9600 {
+        let x = saw.sample();
+        let l = s[0].process(&low, x);
+        let l = s[1].process(&low, l);
+        let h = s[2].process(&high, x);
+        let h = s[3].process(&high, h);
+        let a = s[4].process(&all, x);
+        assert!((l + h - a).abs() < 1e-4, "{} vs {a}", l + h);
+    }
+    let mut s = Svf::default();
+    for hz in [30., 300., 3000.] {
+        assert!(sine_gain(|x| s.process(&all, x), hz).abs() < 0.01);
+    }
+}
+
 #[test]
 fn svf_survives_retuning_every_sample() {
     let mut s = Svf::default();