Dope sheet — dense keyframes

Twelve keyframes alternating ease/linear on a TrackKind::Effect zoom track, in addition to the four standard tracks.

Confirms the dot positioning math holds at high density (no overlap or clipping at the seconds boundary). Also exercises the track-effect visual styling — pink-tinted row background — to validate the per-kind gradient approach.

The playhead is at t=5.1s — not on a frame boundary on purpose, to verify the floating-point positioning.