Transitions: fade starts from the outgoing picture (was an instant cut)

The transition started with no from-frame, so BlendFrame returned the new scene at once.
Start now requests a capture and the frame pump supplies its last output on the next tick.
Test: TransitionFadeTests. Docs: ai.md, TASKS.md.

Not pushed.

ai.md

@@ -1287,3 +1287,12 @@ nothing. COPPA is very unlikely to bind (not child-directed), but note the enfor
 (FTC Sept 2025 *Apitor*: operators are responsible for what third-party components collect) —
 so the answer is a documented inquiry, and ours is "no third-party analytics or ad SDKs, no
 persisted chat or reward payloads."
+
+## Transitions (2026-10-27)
+
+- **Fade starts from the outgoing picture.** `TransitionService.Start` with no "from" frame sets
+  `CaptureRequested`; the frame pump copies its last output at the top of its next tick and calls
+  `SupplyFromFrame`. Before that fix the blend had no starting frame and a scene swap was instant.
+  Guard: `ytLive.Tests/TransitionFadeTests.cs`.
+- **Planned next (see TASKS.md):** transition picker in App Settings, Wipe, Move (persistent layers),
+  brand logo layer, GPU cube (capability-gated).