Class ReplayController

Constructing the controller enters replay: it snapshots the chart's data and viewport and immediately shows startIndex. That is the gesture a replay button makes, and it means the snapshot is taken before anything has moved, so stop() can put the user back exactly where they were.

Halt playback, leaving the playhead (and the chart) where it is.

Start (or re-speed) playback. Calling it on the last bar plays nothing and reports replay:end instead of arming a timer that could never advance.

Jump the playhead to a bar. Out-of-range values clamp to the session.

A seek lands on a complete bar even under intra-bar replay: scrubbing to a half-formed candle is not a position anyone asks for, and it would make the same slider position mean different things on the way past.

Move n steps forward. Stops dead at the end of the session.

A step is one displayed bar normally, and one sub-bar under intra-bar replay, so the transport button means "advance the chart by the smallest amount it can show" in both modes.

Move n steps back. Stops dead at the first bar.

Leave replay: every driven series gets its pre-replay data back and the viewport is restored to the pixel, so a user who exits does not lose their place. Safe to call twice; a later seek/step/play re-enters replay from startIndex.