docs(70): queue the Live Window App overlay layer

Record the creator's Live-only, single-layer scope and distinguish it from the
existing session-scoped Capture Window backdrop pin. Include HWND lifecycle,
protected-window, visual-only, and duplicate-underlay boundaries.

Reference scan: Microsoft HWND capture item API and OBS Window Capture:
https://learn.microsoft.com/en-us/windows/win32/api/windows.graphics.capture.interop/nf-windows-graphics-capture-interop-igraphicscaptureiteminterop-createforwindow
https://obsproject.com/kb/window-capture-sources

HANDOFF.md

@@ -2,7 +2,7 @@
 
 ## State
 
-- `main`, worktree clean. App work is pushed through `b265d42`; TASK 69 queue update is local only.
+- `main`. App code is pushed through `b265d42`; queue-only docs commits are local.
 - **Done:** TASK 35 scene-linked audio, TASK 55 disk hygiene, TASK 65 replay clips, TASK 37 defaults/current split.
 - TASK 37: SQLite keeps an immutable default snapshot beside build-stamped current rows; App Settings can
   restore the staged scene or all scenes, offline only. Webcam identity survives both restore paths.
@@ -12,7 +12,7 @@
 
 ## Next action
 
-- Queue: **40·A/C/D → Move → 9·5 → 14·16/17/18 → 66 → 67 → 68 → 69 → 36**.
+- Queue: **40·A/C/D → Move → 9·5 → 14·16/17/18 → 66 → 67 → 68 → 69 → 70 → 36**.
 - Ask creator before TASK 68 about first-run ON-AIR default; before gold pass, settle the Release
   single-instance mutex behavior.
 - TASK 34 scheduling and TASK 50 recording publish are retired; YouTube owns those surfaces.