0008. Tracks composite in document order

Context

Timelines draw track 1 at the bottom of the UI and composite it at the bottom of the picture; OpenReel's data model lists tracks top-first. A file is read top to bottom.

Decision

The first <Track> in the file is the bottom of the picture; later tracks draw on top. A file reads as "main footage, then overlays, then music", which is how briefs are written.

Consequences

• The timeline UI must list tracks in reverse of the file to look conventional.
• Captions, which are not on a track, draw above everything.