Service: offer "Use Windows default output" (Christopher's request), reusing the app's follower

The service profile could only pick named output devices, so it couldn't "just send
whatever this machine plays and follow the Windows default". The main app already had
exactly that (its DefaultLoopbackSendFollower + ResolveDefaultDeviceId) - so rather
than invent a parallel mechanism (which would silently diverge), this pulls the shared
piece out and has the service reuse it.

- New AudioDefaultFollower: one home for the loopback-send sentinel
  ("__use-default-loopback-send__"), the follower list entry, and the default-endpoint
  resolver. MainForm now references it (its DefaultLoopbackSendFollower and
  ResolveDefaultDeviceId delegate to it) so there is a single definition.
- Service config dialog: the Audio send tab lists "Use Windows default audio device,
  follows Windows changes" as the first output choice. Ticking it persists the same
  sentinel the app uses.
- ServiceSendHost.BuildSendSpecs resolves that sentinel to the LIVE default render
  endpoint (with de-dup against explicitly-ticked devices), never passing the raw
  sentinel through. Because the service re-applies its profile on every device-change
  notification - and OnDefaultDeviceChanged is one of them - it FOLLOWS the default:
  change Windows' default output and the service switches to it within a beat.
- Manual: documents the new option in the service section.

Test: new "Default-output follower" self-test - follower is flagged + shares the app's
sentinel, and the service resolves it to the current Windows default render endpoint
(never leaks the raw sentinel into a capture spec). Gate 44/44.

The ASIO "Rea" devices Ed noticed are real registry drivers (Realtek ASIO + REAPER's
ReaRoute), not injected dummies - we only ever list HKLM\SOFTWARE\ASIO. No code change.

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

src/RemSound.App/SelfTest.cs

@@ -64,6 +64,7 @@ internal static class SelfTest
         RunStep(results, "Lifecycle churn (modes, sources, pan/EQ, send/receive)", LifecycleChurn);
         RunStep(results, "Service app-yield token", ServiceInteractivePresence);
         RunStep(results, "Service sender parity (crypto + Opus frame)", ServiceSenderParity);
+        RunStep(results, "Default-output follower (service follows Windows default)", DefaultOutputFollower);
         RunStep(results, "Service profile isolation (location + hidden from pickers)", ServiceProfileIsolation);
         RunStep(results, "Service send host (headless stream + yield)", ServiceSendHostStream);
         RunStep(results, "Service network presence (reachable + shell teardown)", ServiceNetworkPresenceReachable);
@@ -882,6 +883,29 @@ internal static class SelfTest
     /// AND the fingerprint from the password (a missing fingerprint gets the encrypted stream rejected at
     /// the peer), and apply the send-rate-adjusted Opus frame (the "Small" rate halves it). Guards the
     /// divergences found auditing the service against the main app.</summary>
+    /// <summary>The "Use Windows default output" follower (Christopher's request) must be the SAME shared
+    /// sentinel + resolver the main app uses, and the service must resolve it to the LIVE default render
+    /// endpoint — never pass the raw sentinel through as a device id.</summary>
+    private static string? DefaultOutputFollower()
+    {
+        var choice = AudioDefaultFollower.LoopbackSendChoice();
+        Check(choice.IsDefaultFollower, "the default-output follower must be flagged IsDefaultFollower");
+        Check(AudioDefaultFollower.IsLoopbackSend(choice.DeviceId), "the follower's id must be the loopback-send sentinel");
+        Check(!AudioDefaultFollower.IsLoopbackSend("{0.0.0.00000000}.{abc}"), "a real endpoint id must not be taken for the follower sentinel");
+
+        var p = new Profile { WasapiSendMode = "devices" };
+        p.SelectedWasapiSendOutputs.Add(AudioDefaultFollower.LoopbackSendId);
+        var specs = ServiceSendHost.BuildSendSpecs(p);
+        Check(!specs.Any(s => AudioDefaultFollower.IsLoopbackSend(s.DeviceId)),
+            "the raw follower sentinel must never reach a capture spec — it must be resolved first");
+
+        var expected = AudioDefaultFollower.ResolveDefaultRenderId();
+        if (expected is null) return Skip("no Windows default output device on this box to resolve the follower against");
+        Check(specs.Any(s => s.Kind == CaptureKind.Loopback && s.DeviceId == expected),
+            "the follower must resolve to a loopback spec on the current Windows default output");
+        return "follower flagged + sentinel shared with the app; service resolves it to the live default render endpoint";
+    }
+
     private static string? ServiceSenderParity()
     {
         // The profile deliberately carries the WRONG audio transport (raw PCM, broadcast frame, Standard