feat(screenshare): integrate desktop audio exclusion

src/core/teardown.rs

@@ -305,10 +305,6 @@ pub(super) struct ScreenshareTeardown<C: ChildProcess, G> {
     ///
     /// ⚠️ **LAST FIELD ON PURPOSE** — see the module docs and the struct note.
     ///
-    /// Never read, and that is the design: the guard is held only so that its
-    /// `Drop` runs, and only so that it runs *here*, last. `dead_code` is right
-    /// that nothing reads it and wrong that it does nothing.
-    #[allow(dead_code)]
     echo_cancel: Option<G>,
 }
 
@@ -325,6 +321,13 @@ impl<C: ChildProcess, G> ScreenshareTeardown<C, G> {
         self.host.is_some()
     }
 
+    /// Borrow the session-owned AEC guard without disturbing its load-bearing
+    /// last-field drop order. Phase 8 uses this only to pass the module identity
+    /// to PixelPass while the guard remains owned here.
+    pub(super) fn echo_cancel(&self) -> Option<&G> {
+        self.echo_cancel.as_ref()
+    }
+
     pub(super) fn set_host(&mut self, child: C) {
         self.host = Some(ReapOnDrop::new(child, "screen-share host"));
     }