applyTimelineStackingReorder resolved each z-index change by looking the clip up
in the top-level timelineElements list, but sub-composition children live only
in the expanded list, so the lookup missed them and the reorder silently bailed.
Carry the element's locator (domId/selector/sourceFile) on each z-index change
so the commit resolves the live element directly from the preview DOM. Verified
E2E: dragging a clip inside a sub-composition restacks it and patches the
sub-comp source, while the parent composition is untouched.
Adds timelineEditingHelpers.test.ts (locator-based commit + audio no-op).
Miguel Angel Simon Sierra
82fe779bd8