TASK 4 ship step 4: WASAPI audio capture — NAudio loopback + mic behind an IAudioSource seam, AudioMixer driving AudioLevel while live, pure level meter + WaveToFloat, NAudio.Wasapi 2.2.1 (MIT, notices item 9) — 139 tests passing, 0 warnings

HANDOFF.md

@@ -7,37 +7,50 @@
 
 ## Session state (last updated: 2026-08-12)
 
-- **Branch:** `main`. TASK 4 ship step 3 (encoder + RTMP push) is **built and
-  tested** but **NOT committed** — the working tree is dirty with the encoder
-  (10 new files in `Services/Encoder/` + `ytLive.Tests/FfmpegEncoderTests.cs`) and
-  its memory updates (TASKS.md, ai.md, Services/index.md). Ready to commit when
-  the user says so.
-- **Finished this session:** TASK 4 ship step 3 — the FFmpeg subprocess encoder:
-  `EncoderOptions` + `IFfmpegEncoder`/`FfmpegEncoder` + `IEncoderProcess`/
-  `FfmpegEncoderProcess` + pure `FfmpegArgs`/`FfmpegProgressParser`/
-  `FfmpegEncoderPicker` in `Services/Encoder/`. StartAsync (locate → probe
-  `-encoders` → spawn → stderr loop), SubmitFrameAsync (serialized BGRA stdin),
-  StopAsync (stdin EOF → ffmpeg finalizes; 10s kill watchdog), ProcessFailed on
-  unexpected non-zero exit. Not yet constructed by the app (ship step 5 wiring).
-  Build: **0 warnings**. Tests: **122 passing** (was 112; +10 new).
+- **Branch:** `main`. TASK 4 ship step 4 (WASAPI audio capture) is **built and
+  tested** but **NOT committed** — the working tree is dirty with the audio
+  layer (6 new files in `Services/Audio/` + `ytLive.Tests/AudioMixerTests.cs`,
+  the `NAudio.Wasapi` 2.2.1 package reference, the THIRD-PARTY-NOTICES entry,
+  the `MainViewModel` wiring, the `FfmpegEncoder.cs:139` CS8602 fix) and its
+  memory updates (TASKS.md, ai.md, Services/index.md, HANDOFF.md). Ready to
+  commit when the user says so.
+- **Finished this session:** TASK 4 ship step 4 — the live audio capture layer:
+  `IAudioSource`/`AudioSample` seam, `WasapiLoopbackAudioSource`
+  (`WasapiLoopbackCapture` on the default render device), `WasapiMicAudioSource`
+  (`WasapiCapture`, NAudio device resolved by `FriendlyName` matching
+  `MicSourceName` via a re-read `Func<string?>` provider, default-endpoint
+  fallback), `AudioMixer` (owns both sources, starts/stops with go-live, feeds
+  the pure `AudioLevelMeter` → `MicLevelChanged`), `WaveToFloat` (IEEE-float /
+  PCM16 / extensible). `MainViewModel` constructs the mixer, `BeginGoLive`
+  success → `Start()`, `StopStream` → `Stop()`, `MicLevelChanged` marshalled to
+  the UI thread → `AudioLevel`. Loopback samples are currently dropped (the
+  encoder's AAC mix, ship step 5, consumes them). Build: **0 warnings** (a
+  pre-existing CS8602 in `FfmpegEncoder.cs:139` surfaced during the rebuild and
+  was fixed with `process!`). Tests: **139 passing** (was 122; +17 new audio).
 - **Landmines:**
-  - `ChannelReader.ReadAsync` on a completed channel **throws**
-    `ChannelClosedException` — it does NOT return `null` like a StreamReader EOF.
-    The test fake (`QueuedReader` in `FfmpegEncoderTests.cs`) catches it and
-    returns `null`, or the encoder's stderr loop treats it as a fault and
-    `ProcessFailed` never fires (that's exactly what happened on the first run —
-    see commit history).
-  - The probe process (`FakeEncoderProcess`) must be a *separate* `IEncoderProcess`
-    instance from the encoder process in `Start_*` tests — `StartAsync` calls the
-    factory twice (probe → encoder), and the fake can't simulate both roles at
-    once.
-  - `StopAsync` waits the full `ExitTimeout` if the fake's process doesn't signal
-    exit — fakes must call `SignalExit()` inside `StopAsync`'s stdin-EOF path.
-  - Windows-only: `FfmpegEncoderProcess` sets `UseShellExecute=false` +
-    `RedirectStandardXxx=true` — never spawn with a shell.
-- **Next step:** TASK 4 ship step 4 — WASAPI audio capture (loopback + mic)
-  feeding `AudioLevel` (req 7). Nothing else queued — do not expand the task
-  queue on your own.
+  - `WaveFormatExtensible.SubFormat` is a **`Guid`** in NAudio 2.x, not a
+    `WaveFormatEncoding` — compare it to
+    `NAudio.Dmo.AudioMediaSubtypes.MEDIASUBTYPE_IEEE_FLOAT` (the IEEE-float
+    subtype constant; it lives in the `NAudio.Dmo` namespace, not CoreAudioApi).
+  - Use the **`NAudio.Wasapi` 2.2.1 feature package**, not the `NAudio`
+    meta-package — the wasapi types come from that package (`WasapiCapture` in
+    `NAudio.CoreAudioApi`, `WasapiLoopbackCapture` in `NAudio.Wave`); `NAudio.Core`
+    comes in transitively.
+  - The audio meter is an exponential smoother (0.2 factor) — a single pushed
+    sample only moves 20% toward its RMS. Tests must push repeatedly before
+    asserting a converged level.
+  - Tests never instantiate `MainViewModel` directly except the round-clip
+    integration test (a real `MainWindow`), which never goes live — so the mixer
+    is constructed but never started there; NAudio types are only constructed,
+    never touching devices. Keep it that way.
+  - Mic device resolution must re-read `MicSourceName` at each `Start` (the app
+    persists only the DisplayName, not a device ID).
+  - Windows-only: all capture runs only while live (privacy indicator otherwise).
+
+- **Next step:** TASK 4 ship step 5 — the frame-pipeline wiring
+  (`CameraManager`/`ScreenCaptureManager` → compositor resolver → encoder
+  construction, plus the loopback → encoder AAC mix). Nothing else queued — do
+  not expand the task queue on your own.
 
 - **Secret/DB/port facts live:** OAuth client id/secret in `Helpers/OAuthCredentials.cs`;
   OAuth session token in `Helpers/TokenStore.cs` (DPAPI → `%APPDATA%\ytLlive\ytLlive.auth`);