docs: restructure TASKS.md into a catalog — one file per task in TASKS/

TASKS.md is now the index (status table, open items, research pointer).
33 files: 32 task files + 1 research facts file. The full take-saga
narrative and all design decisions are preserved verbatim; the catalog
makes the queue readable without opening every task body. Schema and
AGENTS.md updated to reflect the new layout.

TASKS/task-19-scene-transitions.md

@@ -0,0 +1,15 @@
+# TASK 19 — Scene transitions → MERGED into TASK 19/23 (Control Surface UX)
+
+> Catalog: [`TASKS.md`](../TASKS.md) — status and requirements live here.
+
+
+**Superseded by the Control Surface UX task.** Cut/Fade/Move transitions, thumbnails, edit mode, left panel two-state — all shipped as one cohesive feature. See the Control Surface UX section in `ai.md`.
+
+### Design decisions
+
+- **Cut is free** — instant switch, no blending, zero CPU cost. This is the default.
+- **Fade is the minimum expectation** — 300ms crossfade is universal. Every streaming tool ships this.
+- **Move is economical** — simple slide animation, no video decoding needed. Good middle ground.
+- **Custom is premium** — media/stinger transitions require video playback. Heavy but expected by mid-tier streamers.
+- Transitions happen in the compositor, not post-encode. The preview sees the same blend as the live output.
+