Speed ramp regions (choreograph per-section speed)

[sauravpanda]

Summary

Define multiple `(startSeconds, endSeconds, speed)` regions on a clip; the player auto-switches speeds as the playhead crosses them.

Acceptance criteria

• `SpeedRegion` model (or reuse `LoopMarker` with `preferredSpeed`)
• Time observer checks `currentTime` against regions; calls `setSpeed` on transitions
• Regions visible on the scrubber as color-tinted bands
• Editor UI: add, drag-resize, delete

Effort

★ — pure Swift on top of existing time-observer infra.