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.

ai.md

@@ -126,18 +126,18 @@ the Socials fediverse-heal roundtrip, AboutHubTests, NotificationAreaIntegration
 GlobalHotkeyTests + HotkeyConfigTests (TASK 20), WebcamMenuGateTests (TASK 26), ChatLayerGateTests
 (TASK 27), BroadcastPullOutTests (TASK 29), DefaultRecordFolder fallback (TASK 30), WebView2ManagerTests
 (TASK 17), RecordingFileTests + OnAirSignTests (TASK 18), SessionTeardownTests (2026-09-01
-rollback) — **~250 total (exact whole-suite count not claimable — the full run hangs and
-`AudioPipelineTests` hangs even STANDALONE (confirmed 2026-09-01 — treat the class as
-un-runnable until the audio channel-init fix lands). Every other class passes individually
-through the Windows dotnet.exe host, including the real-`MainWindow`/RealApp tests — 247 pass
-was the last full-suite number (2026-08-31); since then: the ONE remaining known failure is
-`AudioPipelineTests.Mix_HonorsProviderGains…` (the creator-declared known, suspected WASAPI
-channel declaration/init), `RoundClipInteractionTests.Round_Clip_Corner_Is_Grabbable…` was
-root-caused and FIXED 2026-09-01 (two stale-test layers — namescoped `FindName` +
-`VisualTreeHelper.HitTest` used where `UIElement.InputHitTest` models input; see MyMistakes
-recipe; no product bug), and three tests were ADDED (locator dead-pin 404, EndBroadcast
-close-out, SessionTeardown rollback). "GUI suites can't run from here" was an overstatement —
-per-class Windows-host vstest runs them fine.)**
+rollback) — **ZERO known failures as of 2026-09-01. `AudioPipelineTests` 25/25 green in 26ms —
+the "known failing" `Mix_HonorsProviderGains…` and the class's notorious STANDALONE HANG shared
+one root cause: TASK 22's `_delayedMix` (nullable, never initialized) was dereferenced
+(`delayed.Length` on null) every live-mix tick — a swallowed NRE starved the pipe the tests
+read and flooded startup.log. Test was right, code drifted from the map's own contract
+(loopbackGain = GameAudioVolume — now honored; the "unity because loopback scales with the
+endpoint" assumption was disproven by the creator's 20%-volume meter observation). The former
+RoundClip "known failure" (stale-test layers: namescoped `FindName` + `VisualTreeHelper.HitTest`
+where `UIElement.InputHitTest` models input — see MyMistakes) was fixed the same day.
+Per-class runs through the Windows dotnet.exe host execute the RealApp/MainWindow suites fine;
+only the FULL-suite run still hangs (WASAPI teardown, pre-existing) — and the whole-suite
+"247 total" era count is stale; trust per-class results.**
 
 Reward-event capture (monetization awareness, see the Monetization section) will add its integration
 tests here when it ships: one real chat-poll payload containing all seven reward event types →