Review sweep: fix real bugs found across the service + per-app + settings changes

Parallel code review of this session's changes surfaced several real bugs; fixed the
substantive ones (judgment/cleanup calls held for Ed):

- HIGH Clearing "remembered peers" was resurrected on the next launch: the per-profile
  -> global migration re-ran every startup and re-unioned the profile file's stale copy.
  Added a one-time AppConfig.RememberedPeersMigrated marker so migration runs once and a
  cleared list stays cleared. Self-test pins the clear-then-reload scenario.
- MED PushModeWasapiBackend.Start rethrew on a device-open failure; nothing up the stack
  wraps it, so device churn (a push-eligible single WASAPI source unplugged mid-open)
  could crash the app. Now logs and stays stopped like MixingEngine/ASIO; the device
  watcher / self-heal re-open when a device returns.
- MED Service self-heal: (a) the re-open "no send sources" path left PerformanceMode ON
  and presence up while streaming nothing - now releases cleanly; (b) the 3-attempt
  ladder never refunded, so 3 hiccups over a days-long stint meant permanent silence -
  now refunds when real audio is heard, and resets on a device hot-plug.
- MED ApplyProfile resolved peers (DNS) and enumerated devices INSIDE the gate lock -
  a boot-time DNS hang as SYSTEM stalled Suspend()/yield/self-heal. Moved outside the lock.
- LOW AudioSessionStartWatcher leaked the AudioSessionManager on every Rehook (the WASAPI
  handle-leak fingerprint) - now disposed. New lifecycle self-test.
- LOW stale docstrings (send-all master toggle; ServiceUpdate in-place scheme; Profile
  .SendAllApplications "neither reads nor writes").

Gate: 42/42 (added peers-migration + session-watcher-lifecycle tests).

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

src/RemSound.App/MainForm.cs

@@ -3870,10 +3870,10 @@ public sealed class MainForm : Form
         sessionStartWatcher = null;
     }
 
-    /// <summary>Restores the WASAPI send mode, the "Send all applications" master toggle and the ticked
-    /// app names from a loaded profile. Remembered apps that aren't running right now are seeded into the
-    /// list (ticked, marked "not running") so they resume capture the moment they reappear. On Windows
-    /// too old for process loopback the mode is forced back to devices.</summary>
+    /// <summary>Restores the WASAPI send mode and the ticked app names from a loaded profile (the main
+    /// window's "Send all applications" master toggle was removed 2026-07-16). Remembered apps that aren't
+    /// running right now are seeded into the list (ticked, marked "not running") so they resume capture the
+    /// moment they reappear. On Windows too old for process loopback the mode is forced back to devices.</summary>
     private void RestoreSendModeFromProfile(Profile p)
     {
         suppressSendAppEvents = true;