fix(audio): _delayedMix never initialized — one NRE line behind the 'known failure', the class hang, AND the log flood; loopback gain now honest

The creator-declared known failure (Mix_HonorsProviderGains) and AudioPipelineTests'
standalone hang shared ONE root cause, born in TASK 22: AudioMixer.FillAndMix
dereferenced _delayedMix (declared float[]? , never assigned) as delayed.Length —
every live-mix tick NRE'd before the pipe write, so NO audio ever reached the wire
(tests starved -> hung/fail; app -> swallowed catch logged only ex.Message, 10ms
flood). Now: ctor-allocated + null-check, catch logs WITH stack via AppLog and is
throttled 5s, and a cancelled token breaks out before logging.

Bonus contract fix: AudioGainProvider.LoopbackGain returned unity on a disproven
premise ('loopback scales with endpoint volume'). The creator's tonight observation —
volume at 20%, meter pegged — proves the WASAPI tap is pre-endpoint-volume, so the
mixer must multiply by GameAudioVolume for stream honesty (this is what ai.md always
specified; the 'failing' test encoded the same and was RIGHT).

AudioPipelineTests: 25/25 in 26ms, standalone, no hang. Known-failure count: ZERO.
ai.md tests paragraph rewritten (no more suite-total claims, both ex-'knowns'
explained); TASK 22 regression recorded; MyMistakes: the known-failure-label rules.

Services/Audio/AudioGainProvider.cs

@@ -28,8 +28,12 @@ public sealed class AudioGainProvider
     /// <summary>Mic gain for the live mix (0 = muted).</summary>
     public double MicGain() => _micMuted() ? 0 : _micVolume();
 
-    /// <summary>Loopback (desktop/game) gain for the live mix (0 = muted, 1 = unity).
-    /// Unity because the slider now drives system volume via PushSystemVolume,
-    /// so the WASAPI loopback signal is already scaled by the user's volume choice.</summary>
-    public double LoopbackGain() => _gameMuted() ? 0 : 1.0;
+    /// <summary>Loopback (desktop/game) gain for the live mix (0 = muted).
+    /// Honesty rule (2026-09-01): WASAPI loopback capture does NOT shrink with the
+    /// endpoint volume — the creator dropped system volume to 20% and the captured
+    /// level stayed pegged — so the mixer must multiply by GameAudioVolume itself
+    /// for the stream to follow the knob (the old unity assumption left the stream
+    /// at full desktop volume while the headphones went quiet; ai.md always
+    /// specified loopbackGain = GameAudioVolume — the code had drifted).</summary>
+    public double LoopbackGain() => _gameMuted() ? 0 : _gameVolume();
 }