Fix hard crash when toggling ASIO while sending a specific app

Local checkpoint - NOT for public release.

Repro (Ed): in applications send mode with an app being captured, turning the
ASIO driver off hard-crashed the process. No log and no crash-report file were
written - and MixingEngine.DisposeEntry swallows managed exceptions - which points
to a native access violation, not a managed throw.

Cause: the ASIO toggle rebuilds the capture backend (ApplyAsioMode -> ApplyAudioRuntime
-> ApplySendSources), which disposes the live ProcessLoopbackCapture. The old design
released the WASAPI COM objects from the disposing thread while the capture thread
could still be inside a native GetBuffer call - a classic use-after-free / AV.

Fix: the capture thread now owns the ENTIRE COM lifecycle. It activates, runs, and
releases every COM object itself, in its finally, only after the loop has exited.
StopRecording/Dispose merely signal and join (2s) - they never touch the COM objects.
If the thread ever wedges in a native call we leak it rather than free from outside
(a rare bounded leak beats a hard crash). The thread is also explicitly MTA, and
activation moved onto it, so the async-activation callback can't stall the UI thread.
bufferReady is volatile and only disposed once the thread has genuinely exited.

Also: ApplyAsioMode force-sets the WASAPI send-list visibility for the new mode, which
resurrected the loopback-outputs list in applications mode; re-assert ApplySendModeVisibility
at the end so the correct list stays shown after an ASIO toggle.

New self-test "Per-application capture lifecycle" runs real start/stop/dispose cycles
of the native capture against our own process on hardware - a bad teardown would AV
and fail the gate. Gate: 16/16.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/RemSound.App/SelfTest.cs

@@ -60,6 +60,7 @@ internal static class SelfTest
         RunStep(results, "Per-peer shaping DSP", PeerShapingDsp);
         RunStep(results, "Multi-output fan-out (both lanes)", FanOutToBothOutputs);
         RunStep(results, "Per-application send enumeration", AppSendEnumeration);
+        RunStep(results, "Per-application capture lifecycle", AppSendCaptureLifecycle);
         RunStep(results, "v5 settings and shaping round-trip", V5ConfigRoundTrip);
         RunStep(results, "Profile save and reload", ProfileRoundTrip);
         RunStep(results, "What's-new update marker", WhatsNewMarkerRoundTrip);
@@ -284,6 +285,31 @@ internal static class SelfTest
         return $"enumerated {apps.Count} app(s); process-loopback supported={supported}";
     }
 
+    /// <summary>Exercises the process-loopback capture's real start → run → teardown cycle several times
+    /// against our OWN process, on hardware. This is the regression guard for the ASIO-toggle hard crash:
+    /// a bad COM teardown (releasing objects from the wrong thread / mid-native-call) would take the whole
+    /// test process down with an access violation, failing the gate. SKIP on Windows too old to support
+    /// process loopback.</summary>
+    private static string? AppSendCaptureLifecycle()
+    {
+        if (!RemSound.Sender.ProcessLoopbackCapture.IsSupported)
+            return Skip("process loopback needs Windows 10 build 19041+");
+
+        var pid = Process.GetCurrentProcess().Id;
+        var cycles = 0;
+        for (var i = 0; i < 3; i++)
+        {
+            var capture = new RemSound.Sender.ProcessLoopbackCapture(pid);
+            var frames = 0L;
+            capture.DataAvailable += (_, e) => Interlocked.Add(ref frames, e.BytesRecorded);
+            capture.StartRecording();
+            Thread.Sleep(150);         // let activation + the capture loop run and then be torn down
+            capture.Dispose();          // teardown while the capture thread is live — the crash scenario
+            cycles++;
+        }
+        return $"ran {cycles} start/stop/dispose cycles on pid {pid} with no crash";
+    }
+
     /// <summary>The v5 machine-wide settings and per-peer shaping survive a JSON save/reload: new
     /// AppConfig defaults, the named-peers book, the main tab order, per-peer shaping with parametric
     /// bands, and the new recording default. All in-memory — the real config/profiles aren't touched.</summary>