Filter events from after the stop indicator

Magic-trace doesn't stop recording exactly when the stop indicator is hit. That can be quite confusing, especially if you get unlucky and the stop event is nowhere near the right hand side of the trace.

To fix this, let's filter out events from the trace that happen after the stop indicator returns.

It's conceivable that people will want the current behavior behind a flag, but I weakly think it's not worth the extra UI complexity.

No files or tests are named. Start by locating the stop-indicator handling and the trace event filtering path, then trace how events are retained after the indicator returns. Done means events occurring after the stop indicator are excluded from the displayed trace, with coverage for the trace ending at the indicator.

Written by the indexing model from the issue text.