Scanning views crashes when offset is of a different type

I created a heterogeneous table of albums and tracks.

I scanned tracks and it crashed because one of the results was an album INFO_ item. So I added a filter expression.

Then I scanned and it crashed because the offset was an album INFO_, but scan attempted to convert it to a track key.

Start by reproducing the heterogeneous albums-and-tracks scan described in the issue, including the filter expression that returns an album INFO_ item as the offset. Trace the scan path where that offset is converted to a track key; done means the scan no longer crashes on mismatched offset types and has a regression test.

Written by the indexing model from the issue text.