New variable to determine direction of 'manualAdvance' storyAdvancementMode

Currently, there is no easy way to distinguish between a 'manualAdvance' going to the next page or previous page (forwards or backwards).

There is `storyPageId` and `storyPreviousPageId` to be able to determine direction, but you don't always have all the context necessary at the reporting end.

Perhaps `${manualAdvanceDirection}` could be introduced with values 'left' or 'right' (or something similar).

Start by tracing the storyAdvancementMode manualAdvance path and the existing storyPageId and storyPreviousPageId reporting values. Determine where direction can be exposed and verify that manual advances consistently report a clear forward or backward value; the issue does not name a file or test to run.

Written by the indexing model from the issue text.