docs: fix the three compose examples that failed as written

[a-essawy]

Ran every example payload on the compose page against production. Three did not survive contact:

• Add a layer: text style was missing the required font and color, and used length 5 from the 3 second sea.mp4.
• Effects on a clip: same over-length from sea.mp4.
• Scene-first authoring: the transition sat on the first scene, where the engine ignores it (a scene transition blends INTO it from the previous scene), so the example rendered a hard cut while claiming a crossfade. Prose corrected too, and the overlay length now matches the 7.4s total.

All page examples now run verbatim: verified with six production renders (first render, join, layered title, effects plus animated text, fixed scenes with logo overlay, and the registry example agents copy).