compose blend modes, Ken Burns, and clip volume now render correctly

We rendered every compose feature against the production engine and fixed the four that did not behave as documented.

• blendMode now applies. All 7 non-normal modes (multiply, screen, overlay, darken, lighten, add, difference) previously fell back to normal compositing.
• transform.animateTo now animates over the clip’s real duration. Ken Burns moves used to play so slowly they read as a static frame.
• volume on a video clip now works, so you can duck or mute a clip’s own audio under a music track. It was silently ignored.
• Effects on a composition clip (transform, opacity, color, and the rest) now apply. Setting speed on a composition returns a clear validation error instead of doing nothing.

Full parameter reference#

Discovery got the same treatment. Every field in the compose timeline now carries documentation with units, ranges, defaults, and usage notes. It shows up in three places: the compose reference in the docs, the /jobs/types/compose/schema endpoint, and the MCP list_job_types tool, which now accepts a type argument and returns the complete parameter schema for one job type. An agent can pull the full reference before authoring a timeline instead of guessing from an example.