P2 leftovers: every inline dialog now audited + one shared sample clamp (review complete bar peer extraction)

Inline-built dialogs were invisible to the accessibility audit - which is exactly how
mnemonic-less buttons kept slipping through (the single-instance dialog Andre caught,
and now TWO more found by this very change: ManualPeerPrompt's OK/Cancel and
ProfileSaveAsPrompt's Cancel had no mnemonics - both fixed). Each inline dialog's
construction is split from its ShowDialog into a Build seam, and the audit now covers
15 dialog surfaces (was 8): manual peer prompt, quick profile switch, change password,
password manager, profile name prompt, and the service Additional-options window join
the eight Form dialogs. Behaviour unchanged.

The encoder-boundary clamp is now ONE shared rule (Core SampleClamp) instead of three
private copies in MixingEngine / AsioCaptureBackend / PushModeWasapiBackend - and the
ASIO copy's up-to-four interlocked increments per frame became one batched add per
buffer on the RT thread. Pinned by a new self-test (over-range clamps to exactly +/-1
and counts; +/-1 exactly passes untouched).

Gate 61/61.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/RemSound.App/QuickProfileSwitchDialog.cs

@@ -19,8 +19,25 @@ internal sealed class QuickProfileSwitchDialog
     public static string? Show(IReadOnlyList<ProfileEntry> profiles)
     {
         if (profiles.Count == 0) return null;
+        var (dialog, list, chosenPath) = Build(profiles);
+        using (dialog)
+        {
+            dialog.Shown += (_, _) =>
+            {
+                BringToForeground(dialog);
+                list.Focus();
+            };
+            return dialog.ShowDialog() == DialogResult.OK ? chosenPath() : null;
+        }
+    }
 
-        using var dialog = new Form
+    /// <summary>Audit seam: the real dialog, built with a placeholder profile, never shown. Inline-built
+    /// dialogs used to be invisible to the accessibility audit — how missing mnemonics slipped through.</summary>
+    internal static Form BuildForAudit() => Build(new[] { new ProfileEntry("Audit profile", "audit", true) }).Dialog;
+
+    private static (Form Dialog, ListBox List, Func<string?> ChosenPath) Build(IReadOnlyList<ProfileEntry> profiles)
+    {
+        var dialog = new Form
         {
             Text = "Quick profile switch",
             AccessibleName = "Quick profile switch",
@@ -123,14 +140,7 @@ internal sealed class QuickProfileSwitchDialog
             }
         };
 
-        dialog.Shown += (_, _) =>
-        {
-            BringToForeground(dialog);
-            list.Focus();
-        };
-
-        var result = dialog.ShowDialog();
-        return result == DialogResult.OK ? chosenPath : null;
+        return (dialog, list, () => chosenPath);
     }
 
     private static void BringToForeground(Form form)