Dragging a timeline clip (or its right resize edge) past the end of the
video now extends the composition duration on drop, instead of clamping
the clip at the current end. Extend-only and undoable.
- Relax the move/resize horizontal clamps that pinned a clip's end at the
current duration; effectiveDuration now folds in the active drag/resize
preview so the ruler and track width grow live as you drag past the end.
- On drop, extend the root composition data-duration (and the store) when
the clip's new end exceeds it, via a shared extendRootDurationInSource
helper extracted from the block installer (now the single owner of that
logic). An extending edit routes through the server persist path since
the SDK setTiming op can't express the root composition's own duration.
