Extract the shared realtime audio graph owner

tests/fixtures/standard-eq-build.txt

@@ -0,0 +1,31 @@
+  function build() {
+    if (st.ctx) return true;
+    if (!Lazy.loaded('feature:eq-core') || !Lazy.loaded('feature:vocal-reducer')) {
+      loadSoundFeatures().then(()=>{apply(); Level.apply();}).catch(error=>toast(error.message));
+      return false;
+    }
+    try {
+      st.ctx = new AC();
+      st.filters = EQ_BANDS.map((b) => {
+        const f = st.ctx.createBiquadFilter();
+        f.type = b.type;
+        f.frequency.value = b.f;
+        f.Q.value = 1.0;
+        return f;
+      });
+      for (let i = 0; i < st.filters.length - 1; i++) st.filters[i].connect(st.filters[i + 1]);
+      // Loudness levelling: gain toward -14 LUFS, then a limiter so a boosted
+      // quiet track can never clip.
+      st.level = st.ctx.createGain();
+      st.limiter = st.ctx.createDynamicsCompressor();
+      st.limiter.threshold.value = -1; st.limiter.knee.value = 0; st.limiter.ratio.value = 20;
+      st.limiter.attack.value = 0.003; st.limiter.release.value = 0.25;
+      st.vocal = VocalReducer.create(st.ctx, st.filters[st.filters.length - 1]);
+      st.widener = EqCore.createWidener(st.ctx, st.vocal.output);
+      st.widener.output.connect(st.level);
+      st.level.connect(st.limiter);
+      st.limiter.connect(st.ctx.destination);
+      return true;
+    } catch { st.ctx = null; return false; }
+  }
+