0005. Sequence tracks by default, anchors for overlays

Context

Absolute start on every element (Diffusion Studio, most code renderers) is easy to render and hard to edit: trimming one clip leaves a gap or an overlap and the author must move everything after it. Magnetic timelines (Final Cut) derive position from order.

Decision

A <Track> is layout="sequence" by default: children play back to back in document order, <Gap> makes a hole, writing start is a lint error. A layout="free" track takes a literal start on every child. Any element may instead anchor to another element (or to "start") with an offset. Transitions are siblings between two clips of a sequence track and overlap them by their duration.

Consequences

• Trimming the hook clip moves everything after it, and the title anchored to the hook moves with it; the model writes no arithmetic.
• duration="fill" lets a bed run to the end of whatever the sequence became.
• The resolver iterates anchor and match-duration dependencies and reports cycles.

Alternatives

Only absolute starts: every edit becomes a cascade of number changes in the file. Only magnetic tracks: music beds and B-roll need absolute placement.