Replaces the scene-index-tick ruler with composer's new track.default
element: clips sized by real duration (frames/fps, not equal-width
boxes), a ruler in genuine elapsed seconds, and a playhead that follows
the selected scene but can be dragged to any exact second within a
clip via click-to-seek.
Also fixes a real crash found while live-testing this: addScene()
built a scene object missing source_image/references, which crashed
the Inspector the moment it tried to draw the new scene.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
