feat(audio): implement phase 3c master protection

test/helpers/protection-fake.mjs

@@ -0,0 +1,10 @@
+// Recording tests inject this explicitly. Production never substitutes a gain for
+// protection when AudioWorklet is missing; DSP tests execute the real processor.
+export const fakeProtectionFactory = {
+  async load() {},
+  create(context) {
+    const node = context.createGain();
+    node.port = { onmessage: null, postMessage() {}, close() {} };
+    return node;
+  }
+};