Tests: headless main-window coverage (all tabs + controls) + fix Alt+L clash

Local checkpoint - NOT for public release. Closes the UI-coverage gap Ed pushed on.

- MainForm gets a `headless` ctor flag (defaults false → real startup path byte-for-byte
  unchanged). When true it builds the WHOLE window — every tab, control and menu — but
  skips the OS touches: global-hotkey registration, the status/device-refresh timers, the
  device-change notifier, and the audio-backend mode switch in ApplyAsioMode. The
  disruptive startup work (Connect + sockets, UPnP, update check) already lives in the
  Shown handler, which never fires when a test constructs the form without showing it — so
  a headless construction is naturally side-effect-free.
- New self-test "Main window coverage": constructs the headless main window and audits the
  lot — accessible names present, Alt mnemonics unique per group, tab order forms no cycle.
  4 tabs, 41 interactive controls.
- It immediately EARNED ITS KEEP: caught a real Alt+L collision — the WASAPI latency and
  ASIO latency labels both claimed Alt+L in the same panel, so in WASAPI+ASIO mode the ASIO
  field was unreachable by its shortcut. Moved ASIO latency to Alt+I; WASAPI keeps Alt+L.

Gate 23/23.

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

src/RemSound.App/SelfTest.cs

@@ -73,6 +73,7 @@ internal static class SelfTest
         RunStep(results, "Diagnostics report privacy", DiagnosticsPrivacy);
         RunStep(results, "Bundled resources present", ResourcesPresent);
         RunStep(results, "Dialog accessibility (names + mnemonics)", AccessibilityAudit);
+        RunStep(results, "Main window coverage (all tabs + controls)", MainWindowCoverage);
 
         var failed = results.Count(r => r.Status == "FAIL");
         var skipped = results.Count(r => r.Status == "SKIP");
@@ -966,6 +967,39 @@ internal static class SelfTest
         return detail;
     }
 
+    /// <summary>Constructs the ENTIRE main window in headless mode (no audio backend, no hotkeys, no
+    /// timers, no sockets — see MainForm's `headless` flag) and audits every tab and control: accessible
+    /// names present, Alt mnemonics unique per group, and the tab order forms no cycle. This is the
+    /// "check all the tabs" coverage — the whole main-window surface, proven on every build.</summary>
+    private static string? MainWindowCoverage()
+    {
+        Form? form;
+        try { form = new MainForm(null, RemSound.Core.Profile.NewBlank(), null, null, headless: true); }
+        catch (Exception ex) { return Skip($"headless MainForm could not be constructed: {ex.GetType().Name}: {ex.Message}"); }
+
+        try
+        {
+            var violations = new List<string>();
+            AuditForm("Main window", form, violations);
+            Check(violations.Count == 0, string.Join("; ", violations));
+
+            var tabs = CountControls(form, c => c is TabPage);
+            var interactive = CountControls(form, c => c is CheckBox or Button or ComboBox or ListBox or TrackBar or TextBox);
+            Check(tabs >= 3, $"the main window's tabs should be present (found {tabs})");
+            Check(interactive >= 15, $"the main window's interactive controls should be present (found {interactive})");
+            return $"audited the whole main window: {tabs} tabs, {interactive} interactive controls — names, mnemonics and tab order clean";
+        }
+        finally { try { form.Dispose(); } catch { /* ignore */ } }
+    }
+
+    private static int CountControls(Control root, Func<Control, bool> predicate)
+    {
+        var n = 0;
+        void Walk(Control p) { foreach (Control c in p.Controls) { if (predicate(c)) n++; Walk(c); } }
+        Walk(root);
+        return n;
+    }
+
     private static void AuditForm(string formName, Form form, List<string> violations)
     {
         var all = new List<Control>();