fix(audio): game meter scales by GameAudioVolume — meter/stream/headphones all follow the one knob

Creator report 2026-09-01: with the desktop slider dragged to 20%, the game bar's
meter stayed pegged yellow/red. Root cause: the mic meter's formula (level x volume)
was cloned to the game bar WITHOUT the x-volume factor, and the clone inherited a
false premise — that WASAPI loopback capture tracks the endpoint volume. Tonight's
observation disproves it (the tap is pre-endpoint-volume), so both the meter (display)
and the mix (AudioGainProvider.LoopbackGain, fixed in 5ead064) must scale it themselves.
ai.md's two 'loopback scales with the slider' sentences corrected in the same pass.

Test seam: VolumePushOverride (internal static, mirrors LayoutPathOverride pattern) so
the test never hijacks the machine's real volume. GameMeterHonestyTests (RealApp +
temp DB): unity -> hot/red, 20% -> ~1/5 width/green, push observed.

ytLive.Tests/GameMeterHonestyTests.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using Microsoft.Data.Sqlite;
+using Xunit;
+using ytLive.Services;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+/// <summary>
+/// Game-bar meter honesty (creator bug 2026-09-01): the WASAPI loopback tap is
+/// PRE-endpoint-volume, so lowering the desktop slider left the meter pegged
+/// yellow/red at 20% — the mic formula scales level × volume, the game clone
+/// dropped the factor. The meter, the stream gain, and the headphones must all
+/// follow the one knob.
+/// </summary>
+[Collection("RealApp")]
+public sealed class GameMeterHonestyTests
+{
+    private readonly RealAppHost _app;
+
+    public GameMeterHonestyTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void Game_Meter_Scales_By_Volume()
+    {
+        _app.Run(Run);
+    }
+
+    private void Run()
+    {
+        var tempDb = Path.Combine(Path.GetTempPath(), $"ytLlive-gamemeter-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
+        var pushed = new List<double>();
+        MainViewModel.VolumePushOverride = v => pushed.Add(v);
+        var window = default(MainWindow);
+        try
+        {
+            using (var schema = new LayoutStore(tempDb)) { }
+            SqliteConnection.ClearAllPools();
+
+            window = new MainWindow();
+            var vm = (MainViewModel)window.DataContext;
+            window.Show();
+
+            // Hot desktop bed pegged at full linear level (ToDisplay ~= 1.0).
+            vm.GameAudioVolume = 1.0;
+            vm.GameAudioLevel = 1.0;
+            Assert.True(vm.GameMeterFillWidth > 230, "unity volume must keep the meter hot");
+            Assert.Equal("#ef4444", vm.GameMeterBrush);
+
+            // Drop the knob to 20% — the captured level does NOT change (the OS
+            // tap is pre-endpoint-volume); the meter must scale it anyway.
+            vm.GameAudioVolume = 0.2;
+            vm.GameAudioLevel = 1.0;
+            Assert.InRange(vm.GameMeterFillWidth, 40, 80);
+            Assert.Equal("#22c55e", vm.GameMeterBrush);
+
+            // And the push really reached the (faked) endpoint — one knob, honest.
+            Assert.Contains(0.2, pushed);
+        }
+        finally
+        {
+            MainViewModel.VolumePushOverride = null;
+            if (window != null) window.Close();
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(tempDb); } catch { /* best-effort */ }
+        }
+    }
+}