Timeline ruler + coordinate system — ED.8

The timeline needs one shared map between project frames and screen pixels — for the ruler, the lanes (ED.9–12), the playhead, and snapping (ED.11). That map is TimelineViewport: a zoom (px_per_frame), a scroll (scroll_frame), and the tick math.

flowchart LR
  F["project frame"] -- "× px_per_frame − scroll" --> PX["pixel x"]
  PX -- "÷ px_per_frame + scroll" --> F
  Z["zoom_at(factor, anchor_px)"] -. keeps anchor frame fixed .-> PX

• frame_to_px / px_to_frame round-trip exactly.
• zoom_at(factor, anchor_px) zooms while holding the frame under the anchor pixel fixed — so zooming centred on the playhead keeps the playhead put (a hard NLE expectation).
• pan_px scrolls, clamped so you can't scroll past either end.
• ruler_ticks emits labeled ticks at a "nice" second interval (1/2/5/10/15/30/60/…s) chosen so labels never crowd — frame-correct at every zoom.

The TimelineRuler component renders a fit-to-width ruler — the full-clip "global progress" view the ticket calls for, decoupled from per-lane zoom — with those tick labels, a reactive playhead bound to the editor status, and click-to-seek (clicked fraction → Seek).