Mic audio UX: realtime read-only meter + mic picker

- Meter is a READ-ONLY realtime level display: fill = Math.Min(1, AudioLevel * MicVolume) (AudioLevel = live input from the future mixer, 0 with no input; MicVolume is gain on ambient noise). While the slider is dragged the bar previews the slider position (PreviewMouseLeftButtonDown/Up + LostMouseCapture -> SetVolumeAdjusting); on release it returns to the live level (bounces to 0 with no input). Clicking the meter does nothing.
- MicVolume drives MicMuted (read-only, muted <=> volume 0): sliding to 0 flips the speaker to the red slashed mute icon, sliding up from 0 clears it; speaker button runs volume to 0 or restores the prior level (_volumeBeforeMute, default 0.8) and flashes the meter to the restored position for ~300ms (BeginVolumeFlash/EndVolumeFlash DispatcherTimer, cancelled if the slider is grabbed).
- MIC label opens MicPickerDialog (WinRT audio-capture device enumeration, mirrors camera picker, no new packages); the chosen voice source name shows left-justified INSIDE the meter bar (fill at 75% opacity so text + ruler markings show through).
- New files: Services/IMicrophoneEnumerator, MicrophoneDeviceInfo, WinRtMicrophoneEnumerator; ViewModels/MicPickerViewModel; MicPickerDialog.
- Docs updated (ai.md, TASKS.md, Services/index.md, ViewModels/index.md); build 0 warnings, 65 tests passing

TASKS.md

@@ -159,7 +159,7 @@ Preview shows the transition too (WYSIWYG). No wipes/slides/LUTs beyond the four
    - **Scenes list:** drag rows to reorder scenes
    - **Sources list:** drag rows to reorder sources (this *is* the z-order) — implemented
 
-### Status: 🔶 In progress — milestone 1 (webcam) shipped; **schema v3 (Ship Branch A) shipped**: multi-scene webcam (singleton `Webcam` + per-scene `WebcamSceneConfig`), right-click border/context menu, static OSB-style borders, 50%-per-dimension webcam size cap, device-swap (`ReleaseAllAsync`); **schema v4**: round→rect restore persisted (`WebcamSceneConfig.RectWidth`/`RectHeight`) + one-time legacy-square 16:9 heal on load — 25 tests passing; **screen backdrop (ship task #1) shipped**: live desktop/game capture as a permanent, non-deletable bottom layer (`Source.IsBackdrop`, schema v5), auto-detecting the full-screen game at launch/focus (else the **primary display** — never assumed monitor 0) via `Win32FullScreenDetector` (now with `GetDisplays()`/`PrimaryMonitorIndex()` for the in-app display picker), content re-designated via the OS `GraphicsCapturePicker` ("Change Capture…") or the in-app "Capture Display" submenu, refcounted/shared capture sessions in `ScreenCaptureManager` mirroring `CameraManager` — 45 tests passing; **schema v6**: `Scene.HasBackdrop`, now **Live-only by policy** — the backdrop is enforced by scene name on every load (`EnforceBackdropPolicy`: Starting/BRB/Chat/Ending never carry one; the one-time v5→v6 backfill covers all four), the scene context-menu "Backdrop" checkbox is gone (policy owns the flag), preview watermark hides when the backdrop renders, capture changed to `WindowsRuntimeMarshal.TryGetDataUnsafe` (the CsWinRT-safe frame-read) + downscale to the 1920×1080 master + 5s-throttled error logging (was flooding `startup.log` with 5 MB of cast errors and burning CPU), round webcam no longer re-rasterizes an `ImageBrush` every frame (Image + EllipseGeometry clip) — the live-mode stutter fix; **the five-scene catalog (`SceneCatalog`)**: Starting/Live/BRB/Chat/Ending is the product — work with less, never more; the (+) button only shows when a canonical scene is missing and re-adds it (its menu lists only the missing ones); **webcam-after-session-start fix**: a webcam added to a scene after the camera was already running (e.g. Chat) previously rendered a transparent container — `CameraManager.GetPreviewBitmap` + propagation in `AddWebcamToActiveSceneAsync`/`ReacquireWebcam` now hands the running shared frames to any newly added `WebcamSceneConfig` — 62 tests passing; the **Chat scene's webcam size cap** is raised from 50%-per-dimension (960×540) to half the screen AREA (~1358×764 @16:9, `MaxWebcamWidthFor`/`MaxWebcamHeightFor` keyed by canonical name) so the viewer sees the creator better — 65 tests passing; **"Add Webcam" always opens the camera picker** (deleting one scene's webcam then re-adding used to resurrect the old camera when another scene still used it — `SwapWebcamIdentityAsync` now swaps the app-wide identity if a different camera is chosen, same path as "Change Webcam…"); scenes/sources UI (add/reorder/rename, image + background overlays with move/resize/opacity/reuse) built; **audio UX shipped (UI)**: the bottom-bar footer is now two lines (dropped/duration moved under bitrate/fps), with a centered sound meter (plain filled bar, blank → green → yellow → red, zone markers at 60%/80%) + a MIC chip (volume slider + mute) — the creator's only audio control, desktop/game audio is automatic (KISS rule); the connected YouTube account's avatar/name shows in the top bar next to Start Stream (`SyncConnectedAccount`); the scenes list is content-height now (no dead space before SOURCES); window capture, compositing, encoding pending
+### Status: 🔶 In progress — milestone 1 (webcam) shipped; **schema v3 (Ship Branch A) shipped**: multi-scene webcam (singleton `Webcam` + per-scene `WebcamSceneConfig`), right-click border/context menu, static OSB-style borders, 50%-per-dimension webcam size cap, device-swap (`ReleaseAllAsync`); **schema v4**: round→rect restore persisted (`WebcamSceneConfig.RectWidth`/`RectHeight`) + one-time legacy-square 16:9 heal on load — 25 tests passing; **screen backdrop (ship task #1) shipped**: live desktop/game capture as a permanent, non-deletable bottom layer (`Source.IsBackdrop`, schema v5), auto-detecting the full-screen game at launch/focus (else the **primary display** — never assumed monitor 0) via `Win32FullScreenDetector` (now with `GetDisplays()`/`PrimaryMonitorIndex()` for the in-app display picker), content re-designated via the OS `GraphicsCapturePicker` ("Change Capture…") or the in-app "Capture Display" submenu, refcounted/shared capture sessions in `ScreenCaptureManager` mirroring `CameraManager` — 45 tests passing; **schema v6**: `Scene.HasBackdrop`, now **Live-only by policy** — the backdrop is enforced by scene name on every load (`EnforceBackdropPolicy`: Starting/BRB/Chat/Ending never carry one; the one-time v5→v6 backfill covers all four), the scene context-menu "Backdrop" checkbox is gone (policy owns the flag), preview watermark hides when the backdrop renders, capture changed to `WindowsRuntimeMarshal.TryGetDataUnsafe` (the CsWinRT-safe frame-read) + downscale to the 1920×1080 master + 5s-throttled error logging (was flooding `startup.log` with 5 MB of cast errors and burning CPU), round webcam no longer re-rasterizes an `ImageBrush` every frame (Image + EllipseGeometry clip) — the live-mode stutter fix; **the five-scene catalog (`SceneCatalog`)**: Starting/Live/BRB/Chat/Ending is the product — work with less, never more; the (+) button only shows when a canonical scene is missing and re-adds it (its menu lists only the missing ones); **webcam-after-session-start fix**: a webcam added to a scene after the camera was already running (e.g. Chat) previously rendered a transparent container — `CameraManager.GetPreviewBitmap` + propagation in `AddWebcamToActiveSceneAsync`/`ReacquireWebcam` now hands the running shared frames to any newly added `WebcamSceneConfig` — 62 tests passing; the **Chat scene's webcam size cap** is raised from 50%-per-dimension (960×540) to half the screen AREA (~1358×764 @16:9, `MaxWebcamWidthFor`/`MaxWebcamHeightFor` keyed by canonical name) so the viewer sees the creator better — 65 tests passing; **"Add Webcam" always opens the camera picker** (deleting one scene's webcam then re-adding used to resurrect the old camera when another scene still used it — `SwapWebcamIdentityAsync` now swaps the app-wide identity if a different camera is chosen, same path as "Change Webcam…"); scenes/sources UI (add/reorder/rename, image + background overlays with move/resize/opacity/reuse) built; **audio UX shipped (UI)**: the bottom-bar footer is now two lines (dropped/duration moved under bitrate/fps), with the mic's sound meter + mute button + volume slider grouped CENTERED on the footer's top line, beneath the preview panel (meter: 288px, muted slate track with ruler graduations + muted yellow/red zone tints, green→yellow→red fill; mute = speaker icon → red do-not-symbol when muted, and the slider and speaker stay in sync (volume 0 ⇔ muted — sliding off flips the speaker to muted, sliding up from 0 clears it); mic volume defaults to 80%, muting zeroes the meter and restores the prior volume on unmute (which flashes the meter to the restored position ~300ms before it returns to the live level); the meter is a READ-ONLY realtime level display (fill = live level × volume — volume is a gain on ambient noise; while the slider is dragged the bar previews the slider position and bounces back to the live level on release, which is 0 with no input — clicking the meter does nothing), clicking the MIC label opens a microphone picker whose chosen source shows left-justified inside the meter bar (fill at 75% opacity so the name + ruler markings show through); slim dimensional slider — gradient track/fill, gloss-sphere thumb; the old flat pink 18px-filled one is gone), everything else on line 2 (bitrate/fps/dropped/duration/health left, quality + gear right) — the creator's only audio control, desktop/game audio is automatic (KISS rule); the connected YouTube account's avatar/name shows in the top bar next to Start Stream (`SyncConnectedAccount`); the scenes list is content-height now (no dead space before SOURCES); window capture, compositing, encoding pending
 
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