feat(host): build desktop audio exclusion foundation

src/host/taint/mod.rs

@@ -34,7 +34,12 @@
 //!    *is* a real Link whose output node is the sink node itself (measured).
 //!    A port-granular walk would need a synthetic edge; a node-granular one
 //!    does not.
-//! 3. **Owner bridges** — the intra-process hop the graph cannot see. See
+//! 3. **Hardware-device bridges** — a passive sink can feed a passive source
+//!    on the same physical Device through a mixer/loopback path that PipeWire
+//!    does not expose as a Link. The observer positively classifies both
+//!    terminals and retains their shared `device.id`; the walk conservatively
+//!    adds `sink → source` for that one Device.
+//! 4. **Owner bridges** — the intra-process hop the graph cannot see. See
 //!    [`owner`]; this is the hard one.
 //!
 //! ## Stickiness
@@ -784,6 +789,40 @@ fn downstream_edges(snapshot: &GraphSnapshot, unresolved_input: &mut BTreeSet<Se
             _ => {}
         }
     }
+
+    // A physical sound device may route playback back into capture in its
+    // own mixer/firmware without publishing a PipeWire Link (HDA "Stereo
+    // Mix", USB loopback channels, vendor DSPs). Control-name inspection is
+    // neither portable nor proof of absence, so v1 fails closed: once a
+    // passive hardware sink is tainted, passive capture terminals exported
+    // by the same Device are downstream too.
+    //
+    // Both guards are load-bearing. `session_device` limits this to the
+    // observer's positive hardware-terminal allowlist, so an app-associated
+    // filter cannot invent a bridge. `device_id` limits it to one physical
+    // Device, so the shared WirePlumber client does not fuse every card.
+    let hardware_outputs: Vec<(Serial, snapshot::GlobalId)> = snapshot
+        .nodes()
+        .filter(|node| {
+            node.props.session_device && matches!(node.role, MediaRole::Sink | MediaRole::Duplex)
+        })
+        .filter_map(|node| node.props.device_id.map(|id| (node.serial, id)))
+        .collect();
+    let hardware_inputs: Vec<(Serial, snapshot::GlobalId)> = snapshot
+        .nodes()
+        .filter(|node| {
+            node.props.session_device && matches!(node.role, MediaRole::Source | MediaRole::Duplex)
+        })
+        .filter_map(|node| node.props.device_id.map(|id| (node.serial, id)))
+        .collect();
+    for (from, output_device) in hardware_outputs {
+        for &(to, input_device) in &hardware_inputs {
+            if from != to && output_device == input_device {
+                edges.entry(from).or_default().push(to);
+                receivers.insert(to);
+            }
+        }
+    }
     for targets in edges.values_mut() {
         targets.sort_unstable();
         targets.dedup();