docs(memory): split capture (backdrop, webcam, effects, vertical tier) out of ai.md

Unit 2 of slimming the mandatory read. Verbatim move again - the four capture
blocks leave ai.md unchanged in content, and one stub carries the law.

- docs/ai/capture.md: 'Screen backdrop capture', 'Webcam capture', 'Webcam
  background effects' and 'Vertical (9:16) tier camera placement', byte-
  identical, grouped under backdrop / webcam / vertical headings, with a
  'Related in ai.md' footer.
- ai.md stub keeps: the locked-Background model and NormalizeBackgrounds;
  ActiveBackgroundImage is fed by static art only (2026-08-23 regression);
  window:/picker: keys are session-scoped and a live pin beats auto-detect;
  the settled WinRT interop paths (TryGetDataUnsafe, from-DXGIDevice); the
  VideoFrame seam, own-catch subtype ladder and no fallback camera; one app-
  level webcam identity with live-lock attainability; CameraManager as the one
  effects processing point with per-session mask smoothing; geometry-only
  vertical tier and the model-level rect lock.
- docs/ai/index.md gains the capture.md row; Services/index.md (2) and
  TASKS/task-38-window-backdrop.md repointed.

ai.md 1,975 -> 1,558 lines. No source touched; no push.

TASKS/task-38-window-backdrop.md

@@ -63,7 +63,7 @@ change to the file). This is the fix for the "committed syntax error" landmine.
 ## Model / naming facts
 
 - `Source.CaptureKey` is persisted (`window:<hwnd>`, `monitor:<n>`, `picker:<displayname>`);
-  `Source.IsBackground` marks the one Background per scene. See ai.md §Screen backdrop capture.
+  `Source.IsBackground` marks the one Background per scene. See [`docs/ai/capture.md`](../docs/ai/capture.md) → "Screen backdrop capture".
 - `WindowInfo` (Models) — `Title`, `ProcessName`, `Hwnd` (long), `ProcessId`;
   `Label => "{Title} — {ProcessName}"`; `CaptureKey => $"window:0x{Hwnd:X}"`.
 - `IWindowEnumerator` (Services) — `IReadOnlyList<WindowInfo> Enumerate()`; seam like