Lock-screen service: spike + headless send host + app-yield coordination

Local checkpoint - NOT for public release. First increment of the send-only
Windows service feature (design in memory/project_remsound_service).

THE SPIKE PASSED: the send engine runs fully headless (no window, no message
pump) and streams, proven by a real self-test — the one genuine unknown that
gated the whole feature. Also proves the app-yield model end to end.

What's in this increment (all headless, all tested, 19/19 gate):
- AppConfig: ServiceProfileName + ServiceLoggingEnabled (machine-wide).
- InteractivePresence (Core): the cross-session app-yield token. App holds a
  Global\ mutex for its lifetime; the service checks it and yields while an
  interactive app is present, resuming when it closes OR crashes (OS frees the
  mutex). Name-parameterised internal seams for isolated testing.
- ServiceSendHost (App): loads a send-only profile and streams it to its peers,
  WASAPI-only, no ASIO/receive. ApplyProfile/Suspend/Resume + a RunLoop that
  drives them from the presence token with a settle delay. v1 sends to direct
  peer addresses (LAN/port-forwarded); NAT/relay discovery stays the app's job.
- Program.cs: the interactive app now acquires the presence token at startup so
  a future service yields to it.
- Tests: "Service app-yield token" (held=present, released=absent) and "Service
  send host (headless stream + yield)" — streams a captured device to a local
  receiver over loopback, verifies start/suspend/resume, then drives the full
  RunLoop against the token (held=suspended, released=resumes-and-flows).

Still to come (later increments): the --run-service entry + Windows-service
registration, the Service menu, the 3-tab config dialog, updater integration,
docs. None user-facing yet, so nothing deployed.

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

src/RemSound.App/MainForm.cs

@@ -113,7 +113,7 @@ public sealed class MainForm : Form
     // programmatically repopulate them (mode switch, profile apply, reconcile) so those handlers
     // don't fire MarkProfileDirty or trigger re-entrant rebuilds on our own writes.
     private bool suppressSendAppEvents;
-    // The two sendModeList rows, in order. Index 0 = whole sound devices (classic), 1 = applications.
+    // The two sendModeList rows, in order. Index 0 = whole audio devices (classic), 1 = applications.
     private const int SendModeDevicesIndex = 0;
     private const int SendModeApplicationsIndex = 1;
     // ASIO-side lists. Always present in the form but hidden when ASIO is disabled. The two
@@ -3299,7 +3299,7 @@ public sealed class MainForm : Form
         sendCheckboxPanel.Controls.Add(sendMyAudioCheckbox);
         panel.Controls.Add(sendCheckboxPanel, 1, 6);
 
-        // Row 7: "how to send WASAPI audio" chooser — sound devices (the classic loopback list) or
+        // Row 7: "how to send WASAPI audio" chooser — audio devices (the classic loopback list) or
         // specific applications. Sits right after "Send my audio". Built manually (like the ASIO
         // driver row) so we keep the label reference to collapse the whole row on old Windows.
         BuildSendModeRow(panel, 7);
@@ -3338,7 +3338,7 @@ public sealed class MainForm : Form
         sendModeLabel.Click += (_, _) => FocusControl(sendModeList);
         sendModeList.AccessibleName = "How to send WASAPI audio (Alt+6)";
         sendModeList.Items.Clear();
-        sendModeList.Items.Add("Send whole sound devices");   // SendModeDevicesIndex
+        sendModeList.Items.Add("Send whole audio devices");   // SendModeDevicesIndex
         sendModeList.Items.Add("Send specific applications");  // SendModeApplicationsIndex
         sendModeList.SelectedIndex = SendModeDevicesIndex;
         panel.Controls.Add(sendModeLabel, 0, row);